Manganese-cobalt composite hydroxide and process for producing same, positive electrode active material and process for producing same, and non-aqueous electrolyte secondary battery

ABSTRACT

A positive electrode active material for non-aqueous electrolyte secondary batteries that can achieve a high output characteristic and a high battery capacity when used in a positive electrode of a battery and that can achieve a high electrode density, and a non-aqueous electrolyte secondary battery that uses such a positive electrode active material and can achieve a high capacity and a high output. A lithium-manganese-cobalt composite oxide includes plate-shaped secondary particles each obtained by aggregation of a plurality of plate-shaped primary particles caused by overlapping of plate surfaces of the plate-shaped primary particles, wherein a shape of the primary particles is any one of a spherical, elliptical, oval, or a planar projected shape of a block-shaped object, and the secondary particles have an aspect ratio of 3 to 20 and a volume-average particle size (Mv) of 4 μm to 20 μm as measured by a laser diffraction scattering process.

This application is a Divisional of application Ser. No. 15/322,278,filed Dec. 27, 2016, which is national stage of PCT/JP2015/062531, filedApr. 24, 2015, which claims priority to Japanese Application No.2014-133399, filed Jun. 27, 2014. The entire contents of the priorapplications are hereby incorporated by reference herein in theirentirety.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a manganese-cobalt composite hydroxideand a process for producing the same, a positive electrode activematerial and a process for producing the same, and a non-aqueouselectrolyte secondary battery. More specifically, the present inventionrelates to a manganese-cobalt composite hydroxide as a precursor of alithium-manganese-cobalt composite oxide for use as a positive electrodeactive material in a non-aqueous electrolyte secondary battery such as alithium-ion secondary battery and a process for producing the same, apositive electrode active material and a process for producing the samewith the use of the manganese-cobalt composite hydroxide as a precursor,and a non-aqueous electrolyte secondary battery using the positiveelectrode active material. It is to be noted that this applicationclaims priority based on Japanese Patent Application No. 2014-133399filed in Japan on Jun. 27, 2014.

Description of Related Art

In recent years, there has been a strong demand for development of asmall and lightweight secondary battery having a high energy densitybecause of the widespread use of mobile phones and portable devices suchas laptop computers. Examples of such a secondary battery includelithium-ion secondary batteries using, as a negative electrode, lithium,a lithium alloy, a metal oxide, or carbon, and such lithium-ionsecondary batteries have been actively researched and developed.

A lithium-ion secondary battery using, as a positive electrode activematerial, a lithium-metal composite oxide, especially a lithium-cobaltcomposite oxide, can achieve a 4 V-class high voltage, and is thereforeexpected to serve as a battery having a high energy density. For thisreason, the commercialization of such a lithium-ion secondary batteryhas been accelerated. Many attempts have hitherto been made to develop abattery using a lithium-cobalt composite oxide to achieve excellentinitial capacity characteristic and cycle characteristic, and as aresult, various results have already been obtained.

Major examples of a positive electrode active material that havehitherto been proposed include a lithium-cobalt composite oxide (LiCoO₂)that is relatively easily synthesized, a lithium-nickel composite oxide(LiNiO₂) or a lithium-nickel-cobalt-manganese composite oxide(LiNi_(1/3)Co_(1/3)Mn_(1/3)O₂) using nickel cheaper than cobalt, and alithium-manganese composite oxide (LiMn₂O₄) using manganese, andspherical or almost spherical particles thereof, which are easilysynthesized, are mainly used.

The main characteristics of a battery using such a positive electrodeactive material are a capacity and a power density. Particularly, abattery for hybrid cars, whose demand has significantly increased inrecent years, is required to have a high power density.

The power density of a secondary battery can be increased by, forexample, reducing the thickness of an electrode film used in thebattery. For example, a battery for hybrid cars uses an electrode filmhaving a thickness of about 50 μm. This is because the migrationdistance of lithium ions in the battery for hybrid cars can be reducedby reducing the thickness of the electrode film. A positive electrodeactive material for use in such a thin electrode film may penetrate theelectrode film, and is therefore limited to small-diameter particleshaving a uniform particle size. In the case of an electrode film of abattery for hybrid cars, particles having a particle size of about 5 μmare used.

However, the use of such small-diameter particles in an electrode filmhas a drawback that an electrode density is reduced so that a volumeenergy density is reduced which is an important characteristic inaddition to a power density.

Patent Document 1: JP 2012-84502 A SUMMARY OF THE INVENTION

The way to overcome such a trade-off is, for example, to change theshape of positive electrode active material particles that are generallyspherical or almost spherical, more specifically, to form positiveelectrode active material particles into a plate shape. By formingpositive electrode active material particles into a plate shape, thesurface area of the positive electrode active material particles isincreased as compared to that of spherical particles having the samevolume. Further, a high electrode density can be achieved by orientingthe plate-shaped particles when an electrode is formed Further, theorientation of such particles having a high aspect ratio makes itpossible to further reduce the thickness of the electrode, therebyfurther improving the output of a battery.

As such a plate-shaped positive electrode active material particle, forexample, Patent Literature 1 discloses a plate-shaped particle for apositive electrode active material, wherein when t represents athickness (μm) of the particle, d represents a particle size as a sizein a direction orthogonal to a thickness direction defining thethickness t, and d/t represents an aspect ratio, t≤30 and d/t≥2, andwherein primary crystal particles(lithium-intercalation/deintercalation-plane-oriented particles) whose(003) plane is oriented so as to intersect a plate surface of theplate-shaped particle are placed in a dispersed state in an aggregate ofprimary crystal particles (numerous (003)-plane-oriented particles)whose (003) plane is oriented so as to be parallel to the plate surfaceof the plate-shaped particle.

However, even when the lithium intercalation/deinercalation plane isoriented to the outside of the secondary particle as described in PatentDocument 1, an output characteristic is adversely affected when thepositive electrode active material and an electrolyte do notsufficiently come in contact with each other. Further, Patent Document 1describes a rate characteristic, but does not describe a batterycapacity itself that is an important characteristic of a battery.

As described above, it is difficult for the conventional techniques toindustrially achieve a positive electrode active material that can forma thin electrode film having a high electrode density and that canachieve a high capacity and an excellent output characteristic.

In the light of such a problem, it is an object of the present inventionto provide a positive electrode active material for non-aqueouselectrolyte secondary batteries that can form a thin electrode film, canachieve a high output characteristic and a high battery capacity whenused in a positive electrode of a battery, and can achieve a highelectrode density, and a non-aqueous electrolyte secondary battery thatuses such a positive electrode active material and can achieve a highcapacity and a high output.

It is also an object of the present invention to provide amanganese-cobalt composite hydroxide that is a precursor of a positiveelectrode active material and that makes it possible to provide theabove-described positive electrode active material for non-aqueouselectrolyte secondary batteries.

The present inventors have intensively studied a positive electrodeactive material for non-aqueous electrolyte secondary batteries that hasa shape capable of achieving a high electrode density and amanganese-cobalt composite hydroxide as a precursor of such a positiveelectrode active material. As a result, the present inventors have foundthat a plate-shaped secondary particle can be obtained by aggregation ofa plurality of plate-shaped primary particles caused by overlapping ofplate surfaces of the plate-shaped primary particles due to the controlof the composition of a manganese-cobalt composite hydroxide duringcrystallization and crystallization conditions.

Further, the present inventors have found that a positive electrodeactive material whose shape is derived from the shape of themanganese-cobalt composite hydroxide can be obtained by calcining amixture of the manganese-cobalt composite hydroxide and a lithiumcompound and that a high output characteristic, a high battery capacity,and a high electrode density can simultaneously be achieved. Thesefindings have led to the completion of the present invention.

More specifically, in order to achieve the above objects, the presentinvention is directed to a manganese-cobalt composite hydroxiderepresented by Ni_(x)Co_(y)Mn_(z)M_(t)(OH)_(2+A) (wherein x satisfies0≤x≤0.5, y satisfies 0<y≤0.5, z satisfies 0.35<z<0.8, t satisfies0≤t≤0.1, A satisfies 0≤A≤0.5, x, y, z, and t satisfy x+y+z+t=1, and M isat least one additive element selected from V, Mg, Al, Ti, Mo, Nb, Zr,and W), including plate-shaped secondary particles each obtained byaggregation of a plurality of plate-shaped primary particles caused byoverlapping of plate surfaces of the plate-shaped primary particles,wherein a shape of the plate-shaped primary particles projected in adirection perpendicular to the plate surfaces thereof is any one of aspherical shape, an elliptical shape, an oval shape, and a planarprojected shape of a block-shaped object, and the secondary particleshave an aspect ratio of 3 to 20 and a volume-average particle size (Mv)of 4 μm to 20 μm as measured by a laser diffraction scattering method.

The manganese-cobalt composite hydroxide preferably has a particle sizevariation index represented by [(D90−D10)/Mv] of 0.70 or less, which iscalculated from D90 and D10 of a particle size distribution determinedby a laser diffraction scattering method and the volume-average particlesize (Mv).

Further, in the manganese-cobalt composite hydroxide, an average ofmaximum diameters of the plate-shaped primary particles projected in adirection perpendicular to plate surfaces of the secondary particles ispreferably 1 μm to 5 μm.

Further, in the manganese-cobalt composite hydroxide, at least a cobaltconcentration layer is provided inside the plate-shaped primaryparticles, and the concentration layer preferably has a thickness of0.01 μm to 1 μm.

The present invention is also directed to a process for producing amanganese-cobalt composite hydroxide represented byNi_(x)Co_(y)Mn_(z)M_(t)(OH)_(2+A) (wherein x satisfies 0≤x≤0.5, ysatisfies 0<y≤0.5, z satisfies 0.35<z<0.8, t satisfies 0≤t≤0.1, Asatisfies 0≤A≤0.5, x, y, z, and t satisfy x+y+z+t=1, and M is at leastone additive element selected from V, Mg, Al, Ti, Mo, Nb, Zr, and W),the process including: a nucleation step in which an aqueous solutionfor nucleation that contains a cobalt-containing metal compound so thata content of cobalt to all metal elements contained therein is 90 atom %or more is adjusted to a pH of 12.5 or more on the basis of a liquidtemperature of 25° C. to form plate-shaped crystal nuclei; and aparticle growth step in which a slurry for particle growth containingthe plate-shaped crystal nuclei formed in the nucleation step isadjusted to a pH of 10.5 to 12.5 on the basis of a liquid temperature of25° C. but less than the pH in the nucleation step, and a mixed aqueoussolution containing a metal compound containing at least manganese issupplied to the slurry for particle growth to grow the plate-shapedcrystal nuclei.

In the process for producing a manganese-cobalt composite hydroxide,nucleation in the nucleation step is preferably performed in anon-oxidizing atmosphere whose oxygen concentration is 5 vol % or less,and an ammonia concentration of the slurry for particle growth ispreferably adjusted to 5 g/L to 20 g/L in the particle growth step.

Further, in the process for producing a manganese-cobalt compositehydroxide, the slurry for particle growth is preferably one obtained byadjusting a pH of a plate-shaped crystal nuclei-containing slurrycontaining the plate-shaped crystal nuclei obtained in the nucleationstep.

The present invention is also directed to a positive electrode activematerial for non-aqueous electrolyte secondary batteries including alithium-manganese-cobalt composite oxide represented byLi_(1+u)Ni_(x)Co_(y)Mn_(z)M_(t)O_(2+α) (wherein u satisfies−0.05≤u<0.60, x satisfies 0≤x≤0.5, y satisfies 0<y≤0.5, z satisfies0.35<z<0.8, t satisfies 0≤t≤0.1, a satisfies 0≤α≤0.6, x, y, z, and tsatisfy x+y+z+t=1, and M is at least one additive element selected fromV, Mg, Al, Ti, Mo, Nb, Zr, and W) and having a hexagonal layeredstructure, wherein the lithium-manganese-cobalt composite oxide includesplate-shaped secondary particles each obtained by aggregation of aplurality of plate-shaped primary particles caused by overlapping ofplate surfaces of the plate-shaped primary particles, a shape of theplate-shaped primary particles projected in a direction perpendicular tothe plate surfaces thereof is any one of a spherical shape, anelliptical shape, an oval shape, and a planar projected shape of ablock-shaped object, and the secondary particles have an aspect ratio of3 to 20 and a volume-average particle size (Mv) of 4 μm to 20 μm asmeasured by a laser diffraction scattering method.

The positive electrode active material for non-aqueous electrolytesecondary batteries preferably has a particle size variation indexrepresented by [(D90−D10)/Mv] of 0.75 or less, which is calculated fromD90 and D10 of a particle size distribution determined by a laserdiffraction scattering method and the volume-average particle size (Mv).

Further, in the positive electrode active material for non-aqueouselectrolyte secondary batteries, the lithium-manganese-cobalt compositeoxide is preferably one represented byLi_(1+u)Ni_(x)Co_(y)Mn_(z)M_(t)O_(2+α) (wherein u satisfies 0.40≤u<0.60,satisfies z−x≤u when z−x>0.4, and satisfies u≤z when z<0.6, x satisfies0≤x≤0.5, y satisfies 0<y≤0.5, z satisfies 0.5≤z<0.8, a satisfies0.4≤α<0.6, z and x satisfy z−x<0.6, x, y, z, and t satisfy x+y+z+t=1,and M is at least one additive element selected from V, Mg, Al, Ti, Mo,Nb, Zr, and W).

Further, the positive electrode active material for non-aqueouselectrolyte secondary batteries preferably includes a hexagonal compoundrepresented by a general formula LiMeO₂ and a monoclinic compoundrepresented by a general formula Li₂Me′O₃, wherein Me and Me′ eachrepresent a metal element other than Li.

Further, in the positive electrode active material for non-aqueouselectrolyte secondary batteries, a site occupancy of ions of a metalother than lithium in a 3a site determined by Rietveld analysis of apeak corresponding to the hexagonal lithium-manganese-cobalt compositeoxide in X-ray diffraction is preferably 3% or less, or an orientationindex of a (003) plane corresponding to the hexagonallithium-manganese-cobalt composite oxide determined by X-ray diffractionanalysis is preferably 0.9 to 1.1.

The present invention is also directed to a process for producing apositive electrode active material for non-aqueous electrolyte secondarybatteries including a lithium-manganese-cobalt composite oxiderepresented by Li_(1+u)Ni_(x)Co_(y)Mn_(z)M_(t)O_(2+α) (wherein usatisfies −0.05≤u<0.60, x satisfies 0≤x≤0.5, y satisfies 0<y≤0.5, zsatisfies 0.35<z<0.8, t satisfies 0≤t≤0.1, α satisfies 0≤α<0.6, x, y, z,and t satisfy x+y+z+t=1, and M is at least one additive element selectedfrom V, Mg, Al, Ti, Mo, Nb, Zr, and W) and having a hexagonal layeredstructure, the process including: a mixing step in which amanganese-cobalt composite hydroxide and a lithium compound are mixed toform a lithium mixture; and a calcining step in which the lithiummixture is calcined at a temperature of 650° C. to 1000° C. in anoxidizing atmosphere.

In the process for producing a positive electrode active material fornon-aqueous electrolyte secondary batteries, a ratio (Li/ME) of a numberof lithium atoms (Li) to a sum of numbers of metal atoms other thanlithium (ME) contained in the lithium mixture is preferably 0.95 to 1.6.

Further, the process for producing a positive electrode active materialfor non-aqueous electrolyte secondary batteries preferably furtherincludes, before the mixing step, a heat treatment step in which themanganese-cobalt composite hydroxide is heat-treated at a temperature of300° C. to 750° C. in a non-reducing atmosphere.

Further, in the process for producing a positive electrode activematerial for non-aqueous electrolyte secondary batteries, the oxidizingatmosphere in the calcining step is preferably an atmosphere containing18 vol % to 100 vol % of oxygen.

The present invention is also directed to a non-aqueous electrolytesecondary battery including: a positive electrode; a negative electrode;a non-aqueous electrolyte; and a separator, wherein the positiveelectrode is made of the above-described positive electrode activematerial for non-aqueous electrolyte secondary batteries.

According to the present invention, it is possible to obtain amanganese-cobalt composite hydroxide suitable as a precursor of apositive electrode active material for non-aqueous electrolyte secondarybatteries.

According to the present invention, when the manganese-cobalt compositehydroxide is used as a precursor of a positive electrode active materialfor non-aqueous electrolyte secondary batteries, it is possible toobtain a positive electrode active material that can achieve a highoutput characteristic, a high battery capacity, and a high electrodedensity.

According to the present invention, when the positive electrode activematerial is applied to a non-aqueous electrolyte secondary battery, itis possible to form a thin electrode film and therefore to obtain anon-aqueous electrolyte secondary battery that simultaneously achieves ahigh output characteristic and a high battery capacity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flowchart that illustrates a process for producing amanganese-cobalt composite hydroxide according to the present invention.

FIG. 2 is a flowchart that illustrates a process for producing amanganese-cobalt composite hydroxide according to the present inventiondifferent from the production process illustrated in FIG. 1 in anucleation step.

FIG. 3 is a flowchart that illustrates a process for producing amanganese-cobalt composite hydroxide according to the present inventiondifferent from the production process illustrated in FIG. 1 in aparticle growth step.

FIG. 4 illustrates a schematic sectional view of a coin-type batteryused for battery evaluation.

FIG. 5 is a scanning electron micrograph (observation magnification:1000 times) of a manganese-cobalt composite hydroxide obtained inExample 1.

DETAILED DESCRIPTION OF THE INVENTION

Specific embodiments according to the present invention (hereinafter,referred to as “embodiments of the present invention”) will be describedin detail in the following order with reference to the drawings. It isto be noted that the present invention is not limited to the followingembodiments, and various changes may be made thereto without departingfrom the scope of the present invention.

[1] Manganese-cobalt composite hydroxide and process for producing thesame

[2] Positive electrode active material for non-aqueous electrolytesecondary batteries and process for producing the same

[3] Non-aqueous electrolyte secondary battery

[1] Manganese-Cobalt Composite Hydroxide and Process for Producing theSame

<1-1> Manganese-Cobalt Composite Hydroxide

A manganese-cobalt composite hydroxide according to an embodiment of thepresent invention includes plate-shaped secondary particles eachobtained by aggregation of a plurality of plate-shaped primary particlescaused by overlapping of plate surfaces of the plate-shaped primaryparticles, wherein a shape of the plate-shaped primary particlesprojected in a direction perpendicular to the plate surfaces thereof isany one of a spherical shape, an elliptical shape, an oval shape, and aplanar projected shape of a block-shaped object, and the secondaryparticles have an aspect ratio of 3 to 20 and a volume-average particlesize (Mv) of 4 μm to 20 μm as measured by a laser diffraction scatteringmethod.

The present inventors have studied the filling density of a positiveelectrode active material in a positive electrode and the area ofcontact with an electrolytic solution, and as a result have found thatthe use of a positive electrode active material including plate-shapedsecondary particles each obtained by aggregation of plate-shaped primaryparticles caused by overlapping of plate surfaces of the plate-shapedprimary particles makes it possible to simultaneously achieve animprovement in filling density and an increase in the area of contactwith an electrolytic solution. More specifically, the present inventorshave found that the use of plate-shaped secondary particles eachobtained by aggregation of plate-shaped primary particles, whoseprojected shape in a direction perpendicular to plate surfaces thereofis any one of a spherical shape, an elliptical shape, an oval shape, anda planar projected shape of a block-shaped object, caused by overlappingof the plate surfaces of the plate-shaped primary particles makes itpossible to simultaneously obtain the effect of allowing a sufficientamount of electrolytic solution to enter into the secondary particles,the effect of increasing the area of contact with an electrolyticsolution, and the effect of improving a filling density due to the useof plate-shaped particles.

(Shape and Structure of Particles)

The manganese-cobalt composite hydroxide includes plate-shaped secondaryparticles each obtained by aggregation of a plurality of plate-shapedprimary particles caused by overlapping of plate surfaces of theplate-shaped primary particles, and further, a shape of the plate-shapedprimary particles projected in a direction perpendicular to the platesurfaces thereof is any one of a spherical shape, an elliptical shape,an oval shape, and a planar projected shape of a block-shaped object.Here, the plate surface refers to a surface perpendicular to aprojection direction in which the projected area of the particle ismaximized. Further, the overlapped plate surfaces may be inclined from adirection in which the plate surfaces are parallel to one another sothat the plate-shaped primary particles can be easily aggregatedtogether.

The shape of positive electrode active material particles is derivedfrom the shape of manganese-cobalt composite hydroxide particles as aprecursor thereof (hereinafter, also referred to as “precursorparticles”). Therefore, positive electrode active material particleswhose shape has the same features as the shape of the precursorparticles can be obtained by controlling the shape of the precursorparticles so that plate-shaped secondary particles can be obtained byaggregation of plate-shaped primary particles caused by overlapping ofplate surfaces of the plate-shaped primary particles. It is to be notedthat when conventional small-diameter or plate-shaped precursorparticles are used, positive electrode active material particles whoseshape is derived from the shape of the conventional precursor particlesare obtained, and therefore a positive electrode active material thatwill be described later cannot be obtained.

Further, the secondary particles have an aspect ratio of 3 to 20,preferably 4.5 to 20, more preferably 5 to 15, even more preferably 5 to12, and the volume-average particle size (Mv) of the manganese-cobaltcomposite hydroxide as measured by a laser diffraction scattering methodis 4 μm to 20 μm. Further, in the manganese-cobalt composite hydroxide,an average of maximum diameters of the plate-shaped primary particlesprojected in a direction perpendicular to the plate surfaces thereof(R1) (maximum diameters of the plate-shaped primary particles projectedin a direction perpendicular to the plate surfaces of the secondaryparticles) is preferably 1 μm to 5 μm. If the aspect ratio and Mv thatspecify the shape of the manganese-cobalt composite hydroxide(hereinafter, they are referred to as “shape-specifying values”) exceedtheir respective ranges, the shape-specifying values of the resultingpositive electrode active material may also deviate from theirrespective ranges. Therefore, it is impossible to obtain such an effectof achieving a high output characteristic, a high battery capacity, anda high electrode density as will be described later. Therefore, theshape-specifying values (aspect ratio and Mv) of the manganese-cobaltcomposite hydroxide need to fall within their respective ranges.Further, in order to achieve a higher output characteristic and a higherbattery capacity, R1 is preferably within the above range.

Here, the aspect ratio refers to a ratio (R2/t) of an average of maximumdiameters of the secondary particles projected in a directionperpendicular to the plate surfaces thereof (R2) to an average ofmaximum thicknesses of the secondary particles in a directionperpendicular to the plate surfaces thereof (t). The average of maximumthicknesses (t) is determined by randomly selecting 20 or more of thesecondary particles observable in a direction parallel to the platesurfaces thereof by external observation with a scanning electronmicroscope and measuring and averaging the maximum thicknesses of thesecondary particles. Further, the average of maximum diameters (R2) isdetermined by randomly selecting 20 or more of the secondary particlesobservable in a direction perpendicular to the plate surfaces thereof byexternal observation with a scanning electron microscope and measuringand averaging the maximum diameters of the secondary particles. Then,R2/t is determined from the thus determined average of maximumthicknesses (t) and average of maximum diameters (R2) and is defined asan aspect ratio of the secondary particles. Further, the average ofmaximum diameters of the plate-shaped primary particles (R1) isdetermined by randomly selecting 50 or more of the primary particleswhose entire shape is observable in a direction perpendicular to theplate surfaces thereof and measuring and averaging the maximum diametersof the primary particles in the same manner as in the case of R2.

Since the manganese-cobalt composite hydroxide includes plate-shapedsecondary particles each obtained by aggregation of a plurality ofplate-shaped primary particles caused by overlapping of plate surfacesof the plate-shaped primary particles, voids are sufficiently present inthe secondary particles. Particularly, since the shape of theplate-shaped primary particles projected in a direction perpendicular tothe plate surfaces thereof is any one of a spherical shape, anelliptical shape, an oval shape, and a planar projected shape of ablock-shaped object, the secondary particles have a structure in whichvoids are sufficiently present also in planes parallel to the platesurfaces thereof. Therefore, when the manganese-cobalt compositehydroxide and a lithium compound are mixed and calcined to produce apositive electrode active material, the molten lithium compound spreadsinto the secondary particles so that lithium is sufficiently diffused,which makes it possible to obtain a positive electrode active materialhaving excellent crystallinity. On the other hand, in the case ofplate-shaped secondary particles formed as polycrystals of primaryparticles, voids are not sufficiently present between the primaryparticles, and therefore the molten lithium compound does notsufficiently spread into the secondary particles. The voids inside thesecondary particles constituting the manganese-cobalt compositehydroxide remain even after production of a positive electrode activematerial, and therefore an electrolyte can sufficiently spread intosecondary particles constituting the positive electrode active material.

Further, when the manganese-cobalt composite hydroxide is produced by aprocess for producing a manganese-cobalt composite hydroxide that willbe described later, the primary particles have a cobalt concentrationlayer inside thereof. The secondary particles are formed by growingplate-shaped crystal nuclei formed from a cobalt-containing metalcompound. Therefore, the primary particles of the resultingmanganese-cobalt composite hydroxide have a high concentration layer ofcobalt derived from the plate-shaped crystal nucleus inside thereof.When the plate-shaped crystal nuclei are grown to the extent that thehigh concentration layers are formed, primary particles are grown tohave a desired shape, and further plate-shaped secondary particles areformed by aggregation of the primary particles caused by overlapping ofplate surfaces of the primary particles. On the other hand, when thehigh concentration layer is not present, that is, when the plate-shapedcrystal nuclei are not sufficiently grown, there is a case where theresulting secondary particles do not have a desired shape. However, aslong as being strong enough not to be broken during particle growth andhaving a shape similar to that of plate-shaped crystal nuclei,plate-shaped particles can be used as plate-shaped crystal nuclei so asto be grown to primary particles to form secondary particles. Therefore,when plate-shaped particles having desired composition and shape areseparately prepared, a manganese-cobalt composite hydroxide having nohigh concentration layer is obtained.

In order to grow the primary particles to allow the secondary particlesto have a satisfactory shape, the thickness of the high concentrationlayer is preferably 0.01 μm to 1 μm. If the thickness is less than 0.01μm, there is a case where the plate-shaped crystal nuclei are brokenduring nucleation or particle growth, and therefore the primaryparticles are not sufficiently grown. On the other hand, if thethickness exceeds 1 μm, there is a case where the resulting particles ofthe positive electrode active material have a nonuniform composition orthe grown primary particles do not have a plate shape.

The plate-shaped primary particles having a high aspect ratio can beobtained by growing crystals on both surfaces of each of theplate-shaped crystal nuclei. That is, the primary particles preferablyhave the high concentration layer in the center of the thicknessdirection thereof.

(Composition)

The manganese-cobalt composite hydroxide according to the embodiment ofthe present invention has a composition represented by a general formula(1): Ni_(x)Co_(y)Mn_(z)M_(t)(OH)_(2+A) (wherein x satisfies 0≤x≤0.5, ysatisfies 0<y≤0.5, z satisfies 0.35<z<0.8, t satisfies 0≤t≤0.1, Asatisfies 0≤A≤0.5, x, y, z, and t satisfy x+y+z+t=1, and M is at leastone additive element selected from V, Mg, Al, Ti, Mo, Nb, Zr, and W).

As described above, the manganese-cobalt composite hydroxide contains atleast cobalt. In the general formula (1), y representing a cobaltcontent satisfies 0<y≤0.50, but in order to sufficiently grow theplate-shaped crystal nuclei, y preferably satisfies 0.05≤y≤0.50, andmore preferably satisfies 0.1≤y≤0.50.

When a positive electrode active material is obtained using theabove-described manganese-cobalt composite hydroxide as a raw material,the composition ratio of this composite hydroxide (Ni:Co:Mn:M) ismaintained also in the resulting positive electrode active material.Therefore, the composition ratio of the manganese-cobalt compositehydroxide particles is set to be the same as a desired composition ratioof a positive electrode active material to be obtained. When themanganese-cobalt composite hydroxide has a composition represented bythe general formula (1), a battery using the resulting positiveelectrode active material for non-aqueous electrolyte secondarybatteries can deliver excellent battery performance.

(Particle Size Distribution)

The manganese-cobalt composite hydroxide preferably has a particle sizevariation index represented by [(D90−D10)/Mv] of 0.70 or less, which iscalculated from D90 and D10 of a particle size distribution determinedby a laser diffraction scattering method and the volume-average particlesize (Mv).

The particle size distribution of a positive electrode active materialis greatly influenced by the manganese-cobalt composite hydroxide as aprecursor, and therefore when the manganese-cobalt composite hydroxidecontains fine particles or coarse particles, the resulting positiveelectrode active material also contains such particles. That is, if themanganese-cobalt composite hydroxide has a variation index exceeding0.70 and therefore has a wide particle size distribution, the resultingpositive electrode active material may also contain fine particles orcoarse particles.

Therefore, when the variation index of the manganese-cobalt compositehydroxide is set to 0.70 or less, the variation index of the resultingpositive electrode active material can be made small and a cyclecharacteristic or an output characteristic can be improved. By reducingthe variation index, the characteristics of the positive electrodeactive material can be improved. However, it is difficult to completelyprevent variations in particle size, and therefore the lower limit ofthe variation index is practically about 0.30.

In [(D90−D10)/Mv] representing the particle size variation index, D10refers to a particle size such that, when the number of particles ofeach particle size is cumulatively counted, particles whose cumulativevolume is 10% of the total volume of all the particles have a particlesize smaller than this size. Further, D90 refers to a particle size suchthat, when the number of particles is cumulatively counted in the samemanner as above, particles whose cumulative volume is 90% of the totalvolume of all the particles have a particle size smaller than this size.The volume-average particle size Mv, D90, and D10 can be measured usinga laser diffraction scattering type particle size analyzer.

As described above, the manganese-cobalt composite hydroxide includesplate-shaped secondary particles each obtained by aggregation of aplurality of plate-shaped primary particles caused by overlapping ofplate surfaces of the plate-shaped primary particles, wherein a shape ofthe plate-shaped primary particles projected in a directionperpendicular to the plate surfaces thereof is any one of a sphericalshape, an elliptical shape, an oval shape, and a planar projected shapeof a block-shaped object, and the secondary particles have an aspectratio of 3 to 20 and a volume-average particle size (Mv) of 4 μm to 20μm as measured by a laser diffraction scattering method.

Such a manganese-cobalt composite hydroxide is suitable as a precursorof a positive electrode active material for non-aqueous electrolytesecondary batteries. More specifically, the manganese-cobalt compositehydroxide has the above-described features, and therefore when themanganese-cobalt composite hydroxide is used as a precursor of apositive electrode active material, it is possible to obtain a positiveelectrode active material that has an increased area of contact with anelectrolytic solution and achieves a high filling density. As a result,the manganese-cobalt composite hydroxide can provide a positiveelectrode active material for non-aqueous electrolyte secondarybatteries, which can form a thin electrode film and can achieve a highoutput characteristic, a high battery capacity, and a high electrodedensity.

<1-2> Process for Producing Manganese-Cobalt Composite Hydroxide

A process for producing a manganese-cobalt composite hydroxide accordingto the present invention is intended to produce a manganese-cobaltcomposite hydroxide represented by the following general formula (1) bya crystallization reaction: Ni_(x)Co_(y)Mn_(z)M_(t)(OH)_(2+A) (wherein xsatisfies 0≤x≤0.5, y satisfies 0<y≤0.5, z satisfies 0.35<z<0.8, tsatisfies 0≤t≤0.1, A satisfies 0≤A≤0.5, x, y, z, and t satisfyx+y+z+t=1, and M is at least one additive element selected from V, Mg,Al, Ti, Mo, Nb, Zr, and W).

<1-2-1> Nucleation Step and Particle Growth Step

As shown in FIG. 1, the process for producing a manganese-cobaltcomposite hydroxide includes: a nucleation step in which plate-shapedcrystal nuclei are formed from an aqueous solution for nucleation thatcontains a cobalt-containing metal compound so that a content of cobaltto all metal elements contained therein is 90 atom % or more; and aparticle growth step in which the plate-shaped crystal nuclei formed inthe nucleation step are grown.

Here, in a conventional crystallization process, a nucleation reactionand a particle growth reaction are allowed to simultaneously proceed inthe same vessel. Therefore, in the conventional crystallization process,the resulting composite hydroxide particles are isotropically grown,which makes it difficult to control the shape of the particles.

On the other hand, in the process for producing a manganese-cobaltcomposite hydroxide, the nucleation step in which plate-shaped crystalnuclei are mainly formed by a nucleation reaction and the particlegrowth step in which particle growth is mainly performed on bothsurfaces of each of the plate-shaped crystal nuclei are clearlyseparated. Therefore, the process for producing a manganese-cobaltcomposite hydroxide makes it possible to control the particle shape ofthe resulting manganese-cobalt composite hydroxide. As will be describedlater, the nucleation step and the particle growth step can be separatedby, for example, changing pH used in the nucleation step and pH used inthe particle growth step from each other or changing a reaction vesselused in the nucleation step and a reaction vessel used in the particlegrowth step from each other.

(Nucleation Step)

In the nucleation step, an aqueous solution for nucleation obtained bydissolving a cobalt-containing metal compound in water in apredetermined ratio is adjusted to a pH of 12.5 or higher on the basisof a liquid temperature of 25° C. to form plate-shaped crystal nuclei.

Each of the crystal nuclei corresponds to the above-described cobaltconcentration layer, that is, a layer containing cobalt at a highconcentration, but may contain a metal element other than cobalt. Inorder to grow fine plate-shaped crystal nuclei, the content of cobalt toall metal elements contained in the crystal nuclei is set to 90 atom %or more, preferably 95 atom % or more. In order to sufficiently growfine plate-shaped crystal nuclei, the crystal nuclei are preferably madeof only a hydroxide of cobalt.

In the nucleation step, first, an aqueous solution for nucleation isprepared by dissolving a cobalt-containing metal compound and anothermetal compound in water so that the resulting crystal nuclei can have adesired composition.

Then, the pH of the prepared aqueous solution for nucleation iscontrolled to be 12.5 or higher on the basis of a liquid temperature of25° C. by adding an inorganic alkaline aqueous solution to the aqueoussolution for nucleation. The pH of the aqueous solution for nucleationcan be measured with a common pH meter.

In the nucleation step, the aqueous solution for nucleation is allowedto have a desired composition and the pH of the aqueous solution fornucleation is adjusted to 12.5 or higher at a liquid temperature of 25°C. so that crystal nuclei are grown to have a plate shape, that is,formation of fine plate-shaped crystal nuclei is preferentiallyperformed. As a result, in the nucleation step, fine plate-shapedcrystal nuclei of a cobalt-containing composite hydroxide are formed inthe aqueous solution for nucleation so that a slurry of plate-shapedcrystal nuclei is obtained.

The nucleation step is not limited to one shown in FIG. 1, and may be,for example, one shown in FIG. 2. In the nucleation step shown in FIG.1, the inorganic alkaline aqueous solution is directly added to theaqueous solution for nucleation to form plate-shaped crystal nuclei.

On the other hand, in the nucleation step shown in FIG. 2, an reactionaqueous solution is previously prepared by adding water to the inorganicalkaline aqueous solution so that the pH of the reaction aqueoussolution is adjusted to 12.5 or higher, and the aqueous solution fornucleation is supplied to a reaction vessel, in which the reactionaqueous solution is stirred, to form plate-shaped crystal nuclei whilethe inorganic alkaline aqueous solution is added to maintain the pH ofthe reaction aqueous solution so that a slurry of plate-shaped crystalnuclei is obtained. The process in which the aqueous solution fornucleation is supplied while the pH of the reaction aqueous solution ismaintained is preferred because the pH can be strictly controlled andtherefore plate-shaped crystal nuclei are easily formed.

The nucleation step shown in FIG. 1 or 2 is completed when apredetermined amount of crystal nuclei is formed from the aqueoussolution for nucleation and the inorganic alkaline aqueous solution inthe slurry of plate-shaped crystal nuclei. The determination as towhether a predetermined amount of crystal nuclei has been formed is madebased on the amount of a metal salt added to the aqueous solution fornucleation.

The amount of the nuclei formed in the nucleation step is notparticularly limited. However, in order to obtain manganese-cobaltcomposite hydroxide particles whose shape-specifying values describedabove are within their respective ranges, the amount of the nucleiformed in the nucleation step is preferably 0.1% or more but 2% or less,more preferably 0.1% or more but 1.5% or less of the total amount, thatis, the total amount of metal salts supplied to obtain manganese-cobaltcomposite hydroxide particles.

(Particle Growth Step)

Then, the particle growth step is performed. In the particle growthstep, after the completion of the nucleation step, the slurry ofplate-shaped crystal nuclei in the reaction vessel is adjusted to a pHof 10.5 to 12.5, preferably 11.0 to 12.0 on the basis of a liquidtemperature of 25° C. but lower than the pH in the nucleation step toobtain a slurry for particle growth in the particle growth step. Morespecifically, the pH is controlled by adjusting the amount of theinorganic alkaline aqueous solution supplied. It is to be noted that theparticle growth step shown in FIG. 1 and the particle growth step shownin FIG. 2 are performed in the same manner.

In the particle growth step, a mixed aqueous solution containing atleast a manganese-containing metal compound is supplied to the slurryfor particle growth. In order to obtain a manganese-cobalt compositehydroxide having a desired composition ratio, if necessary, the mixedaqueous solution may contain, in addition to the manganese-containingmetal compound, a nickel-containing metal compound, a cobalt-containingmetal compound, or an additive element-containing metal compound. Theproportion of metals in primary particles grown from the plate-shapedcrystal nuclei as nuclei in the particle growth step is the same as theproportion of metals in the mixed aqueous solution. The proportion ofmetals in the plate-shaped crystal nuclei formed in the nucleation stepis also the same as the proportion of metals in the aqueous solution fornucleation. Therefore, the total of the metal salt used in thenucleation step and the metal salt contained in the mixed aqueoussolution used in the particle growth step is adjusted to achieve adesired proportion of metals in the resulting manganese-cobalt compositehydroxide.

In the particle growth step, the slurry for particle growth is adjustedto a pH of 10.5 to 12.5, preferably 11.0 to 12.0 on the basis of aliquid temperature of 25° C. but lower than the pH in the nucleationstep so that the crystal nuclei growth reaction is more preferentiallyperformed than the crystal nuclei-forming reaction. Therefore, in theparticle growth step, new nuclei are hardly formed in the slurry forparticle growth, but the plate-shaped crystal nuclei are grown toparticles.

In order to form a manganese-cobalt composite hydroxide having acomposition represented by the above general formula (1), the amount ofcobalt contained in the mixed aqueous solution used in the particlegrowth step is smaller than that contained in the aqueous solution fornucleation, and therefore fine plate-shaped crystal nuclei are notformed. Therefore, the plate-shaped crystal nuclei are grown toparticles so that plate-shaped primary particles having acobalt-containing high concentration layer in the center thereof areformed, and further the primary particles are aggregated in anoverlapped manner so that manganese-cobalt composite hydroxide particlesare obtained.

Along with the particle growth caused by supplying the mixed aqueoussolution, the pH of the slurry for particle growth varies, and thereforethe inorganic alkaline aqueous solution is supplied also to the slurryfor particle growth in addition to the mixed aqueous solution to controlthe pH of the slurry for particle growth so that the pH is maintained inthe range of 10.5 to 12.5 on the basis of a liquid temperature of 25° C.

Then, the particle growth step is completed at the time when themanganese-cobalt composite hydroxide particles are grown to have apredetermined particle size and a predetermined aspect ratio. Therelationship between the amount of a metal salt added in each of thenucleation step and the particle growth step and the resulting particlesmay be determined by a preliminary test. This makes it possible toeasily determine the particle size and aspect ratio of themanganese-cobalt composite hydroxide particles from the amount of ametal salt added in each of the steps.

As described above, in the process for producing a manganese-cobaltcomposite hydroxide, plate-shaped crystal nuclei are preferentiallyformed in the nucleation step, and then, in the particle growth step,only the growth of the plate-shaped crystal nuclei into plate-shapedprimary particles and the formation of secondary particles due toaggregation of the plate-shaped primary particles occur, that is, newcrystal nuclei are hardly formed. Therefore, uniform plate-shapedcrystal nuclei can be formed in the nucleation step, and theplate-shaped crystal nuclei can be uniformly grown to particles in theparticle growth step. Further, since the plate-shaped crystal nuclei areuniformly grown to plate-shaped primary particles without causingnucleation, the plate-shaped primary particles are also uniformlyaggregated. Therefore, according to the process for producing amanganese-cobalt composite hydroxide, it is possible to obtain uniformmanganese-cobalt composite hydroxide particles controlled to have anarrow particle size distribution and a desired shape.

It is to be noted that in the process for producing a manganese-cobaltcomposite hydroxide, metal ions are crystallized out as plate-shapedcrystal nuclei or composite hydroxide particles in each of the steps,and therefore the ratio of a liquid component to a metal component ineach of the slurries increases. In this case, the concentration of ametal salt supplied is reduced in appearance, and therefore there is apossibility that composite hydroxide particles are not satisfactorilygrown particularly in the particle growth step.

Therefore, in order to prevent the increase in the liquid component,part of the liquid component contained in the slurry for particle growthis preferably discharged to the outside of the reaction vessel betweenthe time point after the completion of the nucleation step and the timepoint during the particle growth step. More specifically, for example,the plate-shaped crystal nuclei or the manganese-cobalt compositehydroxide particles are settled down by temporarily stopping the supplyof the inorganic alkaline aqueous solution and the mixed aqueoussolution to the slurry for particle growth and the stirring of theslurry for particle growth to discharge the supernatant of the slurryfor particle growth. This makes it possible to increase the relativeconcentration of the mixed aqueous solution in the slurry for particlegrowth. In this case, the manganese-cobalt composite hydroxide particlescan be grown under conditions where the relative concentration of themixed aqueous solution is high, and therefore the particle sizedistribution of the manganese-cobalt composite hydroxide particles canbe made narrower so that the density of secondary particles of themanganese-cobalt composite hydroxide particles can be increased as awhole.

Further, the particle growth steps shown in FIGS. 1 and 2 areadvantageous in that the slurry for particle growth is obtained byadjusting the pH of the slurry of plate-shaped crystal nuclei obtainedin the nucleation step to successively perform the particle growth stepafter the nucleation step, and therefore a quick transition to theparticle growth step can be achieved. Further, the transition from thenucleation step to the particle growth step can be made simply byadjusting the pH of the slurry of plate-shaped crystal nuclei, which isadvantageous in that the pH can be easily adjusted simply by temporarilystopping the supply of the inorganic alkaline aqueous solution or byadding an inorganic acid that is of the same kind as an acidconstituting the metal compound (e.g., by adding sulfuric acid when themetal compound is a sulfate) to the slurry of plate-shaped crystalnuclei.

Here, the particle growth step is not limited to one shown in FIGS. 1and 2, and may be one shown in FIG. 3. In the nucleation step shown inFIG. 3, plate-shaped crystal nuclei may be obtained by directly addingthe inorganic alkaline aqueous solution to the aqueous solution fornucleation as in the case of the nucleation step shown in FIG. 1 or bysupplying the aqueous solution for nucleation to the reaction aqueoussolution with stirring while adjusting the pH of the reaction aqueoussolution as in the case of the nucleation step shown in FIG. 2.

In the particle growth step shown in FIG. 3, a pH-adjusted aqueoussolution whose pH is adjusted by the inorganic alkaline aqueous solutionto be suitable for the particle growth step is prepared separately fromthe slurry of plate-shaped crystal nuclei. Then, the slurry ofplate-shaped crystal nuclei formed by performing the nucleation step inanother reaction vessel, preferably the slurry of plate-shaped crystalnuclei from which part of the liquid component has been removed in theabove-described manner is added to the pH-adjusted aqueous solution toprepare a slurry for particle growth. This slurry for particle growth isused to perform the particle growth step in the same manner as in theparticle growth step shown in FIG. 1 or 2.

According to the process for producing a manganese-cobalt compositehydroxide shown in FIG. 3, the nucleation step and the particle growthstep can more reliably be separated from each other, and therefore theconditions of the reaction aqueous solution in each of the steps can beoptimized for each of the steps. Particularly, the pH of the slurry forparticle growth can be optimized from the start point of the particlegrowth step. Therefore, the manganese-cobalt composite hydroxideobtained in the particle growth step can have a narrower particle sizedistribution can be made uniform and can have a narrower particle sizedistribution.

<1-2-2> Control of pH and Reaction Atmosphere, Particle Size, andAmmonia Concentration

Hereinbelow, the control of pH and reaction atmosphere in each of thesteps, the particle size of manganese-cobalt composite hydroxide, and anammonia concentration will be described in detail.

(pH Control in Nucleation Step)

As described above, in the nucleation step shown in FIGS. 1 to 3, the pHof the aqueous solution for nucleation needs to be controlled to be 12.5or higher on the basis of a liquid temperature of 25° C. If the pH onthe basis of a liquid temperature of 25° C. is lower than 12.5,plate-shaped crystal nuclei are formed but the crystal nuclei themselvesare large, and therefore plate-shaped secondary particles formed byaggregation of plate-shaped primary particles cannot be obtained in thesubsequent particle growth step. On the other hand, although finerplate-shaped crystal nuclei can be obtained at a higher pH, if the pHexceeds 14.0, there is a case where crystallization is difficult tooccur due to the gelation of a reaction liquid or plate-shaped primaryparticles of the manganese-cobalt composite hydroxide are too small.That is, in the nucleation step, the pH of the aqueous solution fornucleation is set to 12.5 or higher, preferably 12.5 to 14.0, morepreferably 12.5 to 13.5 so that plate-shaped crystal nuclei can besatisfactorily formed.

(pH Control in Particle Growth Step)

In the particle growth step, the pH of the slurry for particle growthneeds to be controlled to be 10.5 to 12.5, preferably 11.0 to 12.0 onthe basis of a liquid temperature of 25° C. but lower than the pH in thenucleation step. If the pH on the basis of a liquid temperature of 25°C. is lower than 10.5, the amount of impurities contained in theresulting manganese-cobalt composite hydroxide, for example, anionicconstituent elements contained in metal salts is increased. Further, ifthe pH exceeds 12.5, new crystal nuclei are formed in the particlegrowth step, which deteriorates the particle size distribution. That is,in the particle growth step, the pH of the slurry for particle growth iscontrolled to be 10.5 to 12.5 and lower than the pH in the nucleationstep, which makes it possible to preferentially cause only the growth ofplate-shaped crystal nuclei formed in the nucleation step intoplate-shaped primary particles and the aggregation of the plate-shapedprimary particles and to prevent the formation of new crystal nuclei sothat the resulting manganese-cobalt composite hydroxide is uniform andhas a narrow particle size distribution and a controlled shape. In orderto more clearly separate nucleation and particle growth from each other,the pH of the slurry for particle growth is preferably controlled to belower than the pH in the nucleation step by 0.5 or more, more preferably1.0 or more.

In either of the nucleation step and the particle growth step, thefluctuation range of the pH is preferably plus/minus 0.2 from the setvalue. If the fluctuation range of the pH is large, there is a casewhere nucleation and particle growth are not uniformly performed, andtherefore it is impossible to obtain uniform manganese-cobalt compositehydroxide particles having a narrow particle size distribution.

(Reaction Atmosphere of Nucleation Step)

In the nucleation step, nucleation is preferably performed in anon-oxidizing atmosphere whose oxygen concentration is 5 vol % or less.This makes it possible to prevent the oxidation of cobalt and thereforeto promote the formation of a plate-shaped monocrystal hydroxide,thereby allowing fine plate-shaped crystal nuclei to be formed. When theoxygen concentration is increased, the thickness of the plate-shapedcrystal nuclei tends to increase. If the oxygen concentration exceeds 5vol %, there is a case where fine crystals are aggregated to formspherical or massive nuclei so that plate-shaped crystal nuclei cannotbe obtained. When the plate-shaped crystal nuclei are thicker, theresulting composite hydroxide has a lower aspect ratio. Thenon-oxidizing atmosphere is defined by the oxygen concentration of anatmosphere that is in contact with the aqueous solution during theformation of crystal nuclei or the slurry of plate-shaped crystalnuclei. In order to grow plate-shaped crystal nuclei, the oxygenconcentration is preferably 2 vol % or less, more preferably 1 vol % orless.

(Reaction Atmosphere of Particle Growth Step)

Also when the particle growth step is performed in an oxidizingatmosphere, there is a case where the plate-shaped crystal nuclei arenot grown into dense primary particles so that the resultingmanganese-cobalt composite hydroxide particles are poor in denseness.Therefore, an atmosphere during particle growth, that is, an atmospherethat is in contact with the slurry for particle growth is preferably onewhose oxygen concentration is 10 vol % or less, more preferably onewhose oxygen concentration is 2 vol % or less as in the case of thenucleation step.

Examples of means for maintaining the above-described atmosphere in theinner space of the reaction vessel in each of the steps include flowingof an inert gas such as nitrogen into the inner space of the reactionvessel and bubbling of an inert gas in the reaction liquid.

(Particle Size of Manganese-Cobalt Composite Hydroxide)

The aspect ratio of the formed manganese-cobalt composite hydroxidecorrelates to the size of the crystal nuclei, and therefore can becontrolled by adjusting the pH, the reaction atmosphere, a stirringpower, etc. in the nucleation step. When the plate-shaped crystal nucleiare grown under conditions where oxidation is prevented and a stirringpower is low, primary particles having a large aspect ratio can beobtained, and therefore a manganese-cobalt composite hydroxide having alarge aspect ratio can be obtained. Further, when the plate-shapedcrystal nuclei are grown, large plate-shaped primary particles can beobtained.

Further, the volume-average particle size (Mv) can be controlled by thetime period of the particle growth step. Therefore, manganese-cobaltcomposite hydroxide particles having a desired particle size can beobtained by continuing the particle growth step until a desired particlesize can be achieved. That is, the above-described shape-specifyingvalues can be controlled to fall within their respective ranges bycontrolling the aspect ratio in the nucleation step and regulating theaggregation of primary particles in the particle growth step.

(Ammonia Concentration)

In the particle growth step, ammonia is preferably added as a complexingagent to the slurry for particle growth. In this case, the concentrationof ammonia in the slurry for particle growth is preferably controlled tobe 5 g/L to 20 g/L. Since ammonia functions as a complexing agent, ifthe ammonia concentration is less than 5 g/L, the solubility of metalions cannot be kept constant so that plate-shaped primary particlesgrown from the plate-shaped crystal nuclei are not uniform, which maycause variations in the particle size of the manganese-cobalt compositehydroxide. If the ammonia concentration exceeds 20 g/L, there is a casewhere the solubility of metal ions becomes excessively large, andtherefore the amount of metal ions remaining in the slurry for particlegrowth is increased so that compositional deviation or the like occurs.

Further, if the ammonia concentration fluctuates, the solubility ofmetal ions also fluctuates so that a uniform manganese-cobalt compositehydroxide is not formed. For this reason, the ammonia concentration ispreferably kept at a constant value. For example, the ammoniaconcentration is preferably kept at a desired concentration so that itsfluctuation range is from a value lower by about 5 g/L than a setconcentration to a value higher by about 5 g/L than the setconcentration.

Ammonia is added using an ammonium ion supplier. The ammonium ionsupplier is not particularly limited, and may be, for example, ammonia,ammonium sulfate, ammonium chloride, ammonium carbonate, or ammoniumfluoride.

<1-2-3> Metal Compound to be Used, Reaction Conditions, Etc.

Hereinbelow, the metal compound (metal salt) to be used and conditionssuch as reaction temperature will be described. It is to be noted thatdifferences in these conditions between the nucleation step and theparticle growth step are only the above-described pH and range in whichthe composition of the aqueous solution for nucleation or the mixedaqueous solution is controlled, and the metal compound and conditionssuch as reaction temperature are substantially the same in both thesteps.

(Metal Compound)

As the metal compound, a compound containing a desired metal is used.The compound to be used is preferably a water-soluble compound, andexamples of such a water-soluble compound include metal salts such as anitrate, a sulfate, and a hydrochloride. For example, nickel sulfate,manganese sulfate, and cobalt sulfate are preferably used.

(Additive Element)

The additive element in the general formula (1) (at least one additiveelement selected from V, Mg, Al, Ti, Mo, Nb, Zr, and W) is preferablyadded using a water-soluble compound, and examples of such awater-soluble compound include vanadium sulfate, ammonium vanadate,magnesium sulfate, aluminum sulfate, titanium sulfate, ammoniumperoxotitanate, titanium potassium oxalate, zirconium sulfate, zirconiumnitrate, niobium oxalate, ammonium molybdate, sodium tungstate, andammonium tungstate.

The additive element may be added by adding an additive containing theadditive element to the aqueous solution for nucleation or the mixedaqueous solution so that coprecipitation can be performed in a statewhere the additive element is uniformly dispersed inside themanganese-cobalt composite hydroxide particles.

The additive element may also be added by coating the surface of theresulting manganese-cobalt composite hydroxide with a compoundcontaining the additive element. It is to be noted that when the surfaceis coated with the additive element, the atomic ratio of metal ions ofthe manganese-cobalt composite hydroxide can be made coincident with thefinal composition ratio by reducing the atomic ratio of additive elementions present during the formation of the composite hydroxide bycrystallization by the amount required for coating. Further, the coatingof the surface of the manganese-cobalt composite hydroxide with theadditive element may be performed on particles obtained by heating thecomposite hydroxide.

(Concentration of Mixed Aqueous Solution in Particle Growth Step)

The concentration of the mixed aqueous solution is 1.0 mol/L to 2.6mol/L, preferably 1.5 mol/L to 2.2 mol/L in terms of the total of metalcompounds. If the concentration of the mixed aqueous solution is lessthan 1.0 mol/L, there is a disadvantage that productivity is reduced dueto a reduction in the amount of a crystallized product per reactionvessel.

On the other hand, if the concentration of the mixed aqueous solutionexceeds 2.6 mol/L, it exceeds the saturation concentration at ordinarytemperature and therefore, for example, there is a risk that pipes ofequipment are clogged with re-precipitated crystals.

Further, it is not always necessary to supply, as the mixed aqueoussolution, a mixed aqueous solution containing all the metal compoundsrequired for the reaction to the reaction vessel. For example, whenmetal compounds that form a compound by a reaction caused by mixing areused, their aqueous solutions may be separately prepared so that thetotal concentration of all the metal compound aqueous solutions is 1.0mol/L to 2.6 mol/L, and then simultaneously supplied as individual metalcompound aqueous solutions in a predetermined ratio into the reactionvessel. The aqueous solution for nucleation used in the nucleation stepmay also be prepared as in the case of the mixed aqueous solution.

(Reaction Liquid Temperatures in Nucleation Step and Particle GrowthStep)

The liquid temperature of the reaction liquid during the reaction ineach of the steps is preferably set to 20° C. or higher, particularlypreferably 20° C. to 70° C. When the liquid temperature is less than 20°C., nucleation is likely to occur due to low solubility, which increasesdifficulty in control. Further, in the particle growth step, there is acase where microparticles are formed due to the formation of new nuclei.On the other hand, when ammonia is added, volatilization of ammonia ispromoted if the liquid temperature exceeds 70° C. In this case, it isnecessary to excessively add the ammonium ion supplier to keep theconcentration of ammonia at a predetermined level, which leads to highercosts. When ammonia is not added, the liquid temperature is preferablyset to 40° C. to 70° C. to achieve satisfactory solubility of metalions.

(Inorganic Alkaline Aqueous Solution in Nucleation Step and ParticleGrowth Step)

The inorganic alkaline aqueous solution used to adjust pH is notparticularly limited, and examples thereof include aqueous alkali metalhydroxide solutions such as sodium hydroxide and potassium hydroxide. Inthe case of such an alkali metal hydroxide, the alkali metal hydroxidemay be directly supplied, but is preferably added in the form of anaqueous solution due to the ease of pH control during crystallization.

Further, a process for adding the inorganic alkaline aqueous solution isnot particularly limited, either. For example, the inorganic alkalineaqueous solution may be added using a pump capable of controlling a flowrate, such as a metering pump, while the reaction aqueous solution orthe slurry of plate-shaped crystal nuclei is well stirred so that the pHis kept within a predetermined range.

(Production Equipment)

In the process for producing a manganese-cobalt composite hydroxide, anapparatus is used which is of a type in which a product is not collecteduntil a reaction is completed. For example, a conventionally-usedbatch-type reaction vessel equipped with a stirrer is used. When such anapparatus of a type in which a product is not collected until a reactionis completed is used, unlike a commonly-used continuous crystallizer inwhich a product is collected by overflow, particles having a uniformparticle size and a narrow particle size distribution can be obtainedbecause there is not a problem that growing particles are collectedtogether with an overflow liquid.

Further, when the reaction atmosphere is controlled, an apparatuscapable of controlling an atmosphere, such as a closed apparatus, ispreferably used. When such an apparatus is used, it is possible toeasily obtain the above-described manganese-cobalt composite hydroxideincluding plate-shaped secondary particles each obtained by aggregationof plate-shaped primary particles.

As described above, the process for producing a manganese-cobaltcomposite hydroxide includes: a nucleation step in which an aqueoussolution for nucleation that contains a cobalt-containing metal compoundso that a content of cobalt to all metal elements is 90 atom % or moreis adjusted to a pH of 12.5 or more on the basis of a liquid temperatureof 25° C. to form plate-shaped crystal nuclei; and a particle growthstep in which a slurry for particle growth containing the plate-shapedcrystal nuclei formed in the nucleation step is adjusted to a pH of 10.5to 12.5 on the basis of a liquid temperature of 25° C. but less than thepH in the nucleation step, and a mixed aqueous solution containing atleast a manganese-containing metal compound is supplied to the slurryfor particle growth to grow the plate-shaped crystal nuclei, andtherefore the above-described distinctive manganese-cobalt compositehydroxide can be obtained.

In the process for producing a manganese-cobalt composite hydroxide, thenucleation step and the particle growth step are clearly separatelyperformed, and therefore the above-described distinctivemanganese-cobalt composite hydroxide can be obtained. Further, theproduction process is easily performed, achieves high productivity, andis suitable for industrial-scale production, and therefore itsindustrial value is significantly high.

[2] Positive Electrode Active Material for Non-Aqueous ElectrolyteSecondary Batteries and Process for Producing the Same

(2-1) Positive Electrode Active Material for Non-Aqueous ElectrolyteSecondary Batteries

A positive electrode active material for non-aqueous electrolytesecondary batteries according to an embodiment of the present inventionincludes a lithium-manganese-cobalt composite oxide represented by ageneral formula (2): Li_(1+u)Ni_(x)Co_(y)Mn_(z)M_(t)O_(2+α) (wherein usatisfies −0.05≤u<0.60, x satisfies 0≤x≤0.5, y satisfies 0<y≤0.5, zsatisfies 0.35<z<0.8, t satisfies 0≤t≤0.1, a satisfies 0≤α<0.6, x, y, z,and t satisfy x+y+z+t=1, and M is at least one additive element selectedfrom V, Mg, Al, Ti, Mo, Nb, Zr, and W) and having a hexagonal layeredstructure. This positive electrode active material includes plate-shapedsecondary particles each obtained by aggregation of a plurality ofplate-shaped primary particles of the lithium-manganese-cobalt compositeoxide caused by overlapping of plate surfaces of the plate-shapedprimary particles, wherein a shape of the plate-shaped primary particlesprojected in a direction perpendicular to the plate surfaces thereof isany one of a spherical shape, an elliptical shape, an oval shape, and aplanar projected shape of a block-shaped object, and the secondaryparticles have an aspect ratio of 3 to 20 and a volume-average particlesize (Mv) of 4 μm to 20 μm as measured by a laser diffraction scatteringmethod.

The above composition makes it possible for the positive electrodeactive material for non-aqueous electrolyte secondary batteries to offerexcellent performance. Further, since the positive electrode activematerial includes plate-shaped secondary particles each obtained byaggregation of a plurality of plate-shaped primary particles of thelithium-manganese-cobalt composite oxide caused by overlapping of platesurfaces of the plate-shaped primary particles, the positive electrodeactive material can have an increased area of contact with anelectrolytic solution and can achieve a high filling density due to itsplate shape. Therefore, when this positive electrode active material isused in a positive electrode of a battery, a high output characteristic,a high battery capacity, and a high electrode density can be achieved.

(Composition)

In the positive electrode active material, u representing an excessamount of lithium satisfies −0.05≤u<0.60. If the excess amount oflithium u is less than −0.05, that is, if the lithium content is lessthan 0.95, the reaction resistance of a positive electrode using theresulting positive electrode active material in a non-aqueouselectrolyte secondary battery is increased so that the output of thebattery is reduced.

On the other hand, if the excess amount of lithium u exceeds 0.60, thatis, if the lithium content exceeds 1.60, the initial discharge capacityof a battery having a positive electrode using the resulting positiveelectrode active material is reduced and the reaction resistance of thepositive electrode is also increased.

Further, x representing the nickel content satisfies 0≤x≤0.50. Nickel isan element that contributes to an improvement in initial dischargecapacity. If the value of x exceeds 0.50, thermal stability issignificantly reduced.

Further, y representing the cobalt content satisfies 0<y≤0.5. Cobalt isan element that contributes to an improvement in cycle characteristic.If the value of y exceeds 0.50, a reduction in initial dischargecapacity is significantly reduced. As described above, since themanganese-cobalt composite hydroxide as a precursor used to produce thepositive electrode active material grows from plate-shaped crystalnuclei made of a hydroxide containing at least cobalt, y satisfies 0<y,preferably satisfies 0.05≤y≤0.50, more preferably 0.1≤y≤0.50.

Further, z representing the manganese content satisfies 0.35<z<0.8.Manganese is an element that contributes to an improvement in thermalstability and forms Li₂M′O₃, which will be described later, to improvean initial discharge capacity. If the value of z is 0.8 or more,manganese is eluted into an electrolytic solution during storage at hightemperature or operation of a battery, characteristic deteriorationoccurs.

As shown by the general formula (2), the positive electrode activematerial is more preferably prepared by allowing a lithium-transitionmetal composite oxide to contain an additive element M. When thepositive electrode active material contains the additive element M, thedurability or output characteristic of a battery using such a positiveelectrode active material can be improved. Particularly, when theadditive element M is uniformly distributed in the surface or inside ofthe particles, the entire particles can obtain these effects. Therefore,addition of only a small amount of the additive element M makes itpossible to obtain these effects and prevent a reduction in capacity.

If the ratio t of atoms of the additive element M to all the atomsexceeds 0.1, the amount of metal elements that contribute to a Redoxreaction is reduced, which is disadvantageous in that a battery capacityis reduced. Therefore, the atomic ratio of the additive element M isadjusted to satisfy 0≤t≤0.1.

(Shape and Structure of Particles)

The positive electrode active material uses, as its precursor, theabove-described manganese-cobalt composite hydroxide includingplate-shaped secondary particles each obtained by aggregation of aplurality of plate-shaped manganese-cobalt composite hydroxide primaryparticles, whose shape as projected in a direction perpendicular to theplate surfaces thereof is any one of a spherical shape, an ellipticalshape, an oval shape, and a planar projected shape of a potato-likeblock-shaped object, caused by overlapping of plate surfaces of theplate-shaped primary particles. Therefore, the positive electrode activematerial has the same particle structure as the manganese-cobaltcomposite hydroxide.

In the positive electrode active material having such a structure, voidsare sufficiently present also in the surfaces of the primary particlesin each of the secondary particles as in the case of themanganese-cobalt composite hydroxide. Therefore, such a positiveelectrode active material has a larger specific surface area as comparedto common plate-shaped particles including plate-shaped secondaryparticles formed as polycrystals including primary particles. Further,the individual primary particles have a small particle size, which makesit possible to easily perform lithium insertion/extraction and thereforeto increase the insertion/extraction speed of lithium. Further, thesecondary particles are formed from constituent particles each obtainedby aggregation of the primary particles, which makes it possible tosufficiently spread an electrolyte in the secondary particles. Further,lithium insertion/extraction is performed in gaps or grain boundariesbetween the primary particles, which makes it possible to furtherincrease the insertion/extraction speed of lithium. These effects makeit possible to achieve an output characteristic similar to that whensmall-diameter particles are used, that is, an output characteristicsignificantly improved as compared to that when plate-shaped particlesare used.

On the other hand, since the individual secondary particles aretwo-dimensionally grown by aggregation caused by overlapping of theplate surfaces of the primary particles, gaps between particles such asthose created by filling with small-diameter particles can be reduced byorienting the secondary particles when an electrode is produced byfilling with the secondary particles. This makes it possible to achievea high filling density and a high volume energy density. Further, it isalso possible to reduce the thickness of an electrode film. Therefore,as described above, the use of the positive electrode active materialincluding plate-shaped secondary particles each obtained by aggregationof a plurality of plate-shaped manganese-cobalt composite hydroxideprimary particles caused by overlapping of plate surfaces of theplate-shaped primary particles makes it possible to simultaneouslyachieve a high output characteristic, a high battery capacity, and ahigh electrode density.

The secondary particles constituting the positive electrode activematerial have an aspect ratio of 3 to 20, preferably 4.5 to 20, morepreferably 5 to 15, even more preferably 5 to 12, and the volume-averageparticle size (Mv) of the positive electrode active material as measuredby a laser diffraction scattering method is 4 μm to 20 μm.

If the aspect ratio is less than 3, flatness of the plate shape isdecreased, and therefore a high filling density cannot be achieved byorienting the secondary particles when an electrode is formed. Further,resistance is increased when lithium is diffused to the inside of theparticles so that an output characteristic is reduced. On the otherhand, if the aspect ratio exceeds 20, the particle strength of thesecondary particles is reduced, and therefore the particles are easilycollapsed when a slurry for forming an electrode is kneaded so that theeffects obtained by the plate shape are not sufficiently exerted.Further, the filling density of the secondary particles in an electrodeis also reduced so that a volume energy density is reduced.

If the volume-average particle size (Mv) is less than 4 gaps between thesecondary particles are increased during filling even when the secondaryparticles have a plate shape so that a volume energy density is reduced.Further, the viscosity of a slurry for forming an electrode is increasedduring kneading so that handleability is deteriorated. If thevolume-average particle size (Mv) exceeds 20 μm, lineation occurs whenan electrode film is formed, or a short circuit is caused by penetrationinto a separator. By setting the volume-average particle size (Mv) to 4μm to 20 μm, it is possible to obtain a positive electrode activematerial that can achieve a high volume energy density when used in anelectrode and can prevent lineation when an electrode film is formed ora short circuit caused by penetration into a separator.

Further, in the positive electrode active material, the average ofmaximum diameters of the plate-shaped primary particles projected in adirection perpendicular to the plate surfaces thereof (maximum diametersof the plate-shaped primary particles projected in a directionperpendicular to the plate surfaces of the secondary particles) ispreferably 1 μm to 5 μm. In this case, insertion/extraction of lithiumis performed in gaps or grain boundaries present between theplate-shaped primary particles, which makes it possible to achieve anoutput characteristic similar to that when small-diameter particles areused, that is, an output characteristic significantly improved ascompared to that when plate-shaped particles are used. If the average ofmaximum diameters of the plate-shaped primary particles is less than 1μm, there is a case where gaps between the plate-shaped primaryparticles become too large, and therefore the denseness of the secondaryparticles is reduced so that a satisfactory filling density cannot beachieved. On the other hand, if the average of maximum diameters exceeds5 μm, there is a case where the effects obtained by filling withsmall-diameter particles cannot be sufficiently obtained. Theshape-specifying values (aspect ratio, Mv) and the average of maximumdiameters of the plate-shaped primary particles can be determined in thesame manner as those of the manganese-cobalt composite hydroxide as aprecursor.

(Particle Size Distribution)

The positive electrode active material preferably has a particle sizevariation index represented by [(D90−D10)/Mv] of 0.75 or less, which iscalculated from D90 and D10 of a particle size distribution determinedby a laser diffraction scattering method and the volume-average particlesize (Mv).

When the positive electrode active material has a wide particle sizedistribution, many fine particles having a particle size much smallerthan the average particle size and many coarse particles having aparticle size much larger than the average particle size are present inthe positive electrode active material. If a positive electrode isformed using a positive electrode active material containing many fineparticles, there is a possibility that heat is generated due to a localreaction of the fine particles, and therefore there is a case wheresafety is reduced and a cycle characteristic is deteriorated because thefine particles are likely to be selectively deteriorated. On the otherhand, if a positive electrode is formed using a positive electrodeactive material containing many coarse particles, the coarse particlesreduce a reactive area between an electrolytic solution and the positiveelectrode active material so that there is a case where the output of abattery is reduced due to an increase in reaction resistance. Byreducing the variation index, the characteristics of the positiveelectrode active material can be improved. However, the lower limit ofthe variation index is practically about 0.30.

Therefore, the ratio of fine particles or coarse particles can bereduced by setting the particle size variation index indicating theparticle size distribution of the positive electrode active material,which is represented by [(D90−D10)/Mv], to 0.75 or less. A batteryhaving a positive electrode using such a positive electrode activematerial has a higher level of safety, a higher cycle characteristic,and a higher output. It is to be noted that the average particle size,D90, and D10 are the same as those of the above-described compositehydroxide particles and can be measured in the same manner.

The lithium-manganese-cobalt composite oxide constituting the positiveelectrode active material is preferably one represented byLi_(1+u)Ni_(x)Co_(y)Mn_(z)M_(t)O_(2+α) (wherein u satisfies 0.40≤u<0.60,satisfies z−x≤u when z−x>0.4, and satisfies u≤z when z<0.6, x satisfies0≤x≤0.5, y satisfies 0<y≤0.5, z satisfies 0.5≤z<0.8, α satisfies0.4≤α<0.6, z and x satisfy z−x<0.6, x, y, z, and t satisfy x+y+z+t=1,and M is at least one additive element selected from V, Mg, Al, Ti, Mo,Nb, Zr, and W).

When the positive electrode active material has such a composition, ahexagonal compound represented by a general formula: LiMeO₂, where Merepresents a metal element other than Li, and a monoclinic compoundrepresented by a general formula: Li₂Me′O₃, wherein Me′ represents ametal element other than Li, are formed, which makes it possible toachieve a higher capacity. The reason for this is considered to be thatwhen the positive electrode active material has such a composition, amonoclinic compound represented by Li₂Me′O₃, especially Li₂MnO₃, thatcontributes to a high capacity is formed, and a layered compoundrepresented by LiMeO₂ that causes charge and discharge reactions due tothe insertion and extraction of Li is present around Li₂Me′O₃, andtherefore Li insertion and extraction reactions are promoted even inLi₂Me′O₃, in which Li insertion and extraction reactions are usuallyless likely to occur, so that a battery capacity is increased.Therefore, it is considered that when the ratio of Li₂Me′O₃ is higher, adischarge capacity is higher in terms of theoretical capacity. However,if the ratio of Li₂Me′O₃ is too high, the amount of LiMeO₂ presentaround Li₂Me′O₃ is reduced so that the effect of promoting the insertionand extraction of Li is reduced. Therefore, inactive Li₂Me′O₃ isincreased so that a battery capacity is reduced. Further, it isadvantageous for enhancement of such a promoting effect to increase thecontact interfaces between Li₂Me′O₃ and LiMeO₂. Therefore, the positiveelectrode active material preferably has a structure in which Li₂Me′O₃and LiMeO₂ are finely mixed.

Here, when u representing an excess amount of Li is increased, theamount of Li₂Me′O₃ present in the positive electrode active material isincreased so that the capacity of a battery is increased. For thisreason, u is preferably 0.40 or more. On the other hand, if u exceeds0.60, there is a case where electricity cannot be extracted due to anextreme reduction in activity so that the initial discharge capacity ofthe positive electrode active material is reduced and the reactionresistance of the resulting positive electrode is increased.

Further, when an excess amount of Mn with respect to Ni, that is, “z−x”is more than 0.4, u needs to be equal to or more than (z−x). If u isless than (z−x), there is a case where the amount of Li₂MnO₃ formed isreduced so that a battery capacity is reduced. Further, if the excessamount of Li exceeds the amount of Mn when z representing the amount ofMn is less than 0.6, there is a case where a battery capacity is reduceddue to an increase in the amount of excess Li that does not form Li₂MnO₃with Mn.

At least one of Ni and Co is preferably contained, and x representingthe amount of Ni satisfies 0≤x≤0.5, and y representing the amount of Cosatisfies 0<y≤0.5. If any one of x and y exceeds 0.5, the amount ofLi₂MnO₃ formed is reduced so that a battery capacity is reduced. On theother hand, if both x and y are 0, LiMeO₂ is not formed so that abattery capacity is reduced.

Further, z representing the amount of Mn preferably satisfies 0.5≤z<0.8.If z is less than 0.5, there is a case where Li₂MnO₃ is not sufficientlyformed and unreacted Li is present so that battery characteristics aredeteriorated. On the other hand, if z is 0.8 or more, a spinel phasesuch as LiNi_(0.5)Mn_(1.5)O₄ is formed due to a shortage of Li necessaryfor forming Li₂MnO₃ and LiMnO₂ so that battery characteristics aredeteriorated. In order to prevent the formation of a spinel phase, “x−z”is preferably set to 0.6 or less.

Further, α in the general formula is a value representing an excessamount of oxygen (O), and is preferably in the same range as u to formLi₂Me′O₃ and LiMeO₂.

As described above, in order to achieve a higher capacity, the positiveelectrode active material preferably includes hexagonal LiMeO₂ andmonoclinic Li₂Me′O₃.

Further, in the positive electrode active material, a site occupancy ofmetal ions other than lithium in a 3a site determined by Rietveldanalysis of a peak corresponding to a hexagonal lithium-transition metalcomposite oxide in X-ray diffraction analysis is preferably 3% or less.When the site occupancy in the 3a site is within the above range, cationmixing does not occur in the lithium-manganese composite oxide, andtherefore the lithium-manganese composite oxide has a highcrystallinity, which makes it possible to achieve higher batterycharacteristics, especially a higher charge-discharge capacity or ahigher output characteristic. If crystallinity is low, there is a casewhere metal ions in the 3a site interferes with the migration of lithiumions so that battery characteristic are deteriorated.

Further, the orientation index of a (003) plane corresponding to thehexagonal lithium-transition metal composite oxide constituting thepositive electrode active material determined by X-ray diffractionanalysis is preferably 0.9 to 1.1. The orientation index within theabove range indicates that crystals are randomly arranged in anon-orientation state. When crystals are arranged in a non-orientationstate, it is possible to simultaneously achieve a battery capacity oroutput characteristic influenced by the insertion and extractionperformance of lithium and a cycle characteristic or safety influencedby the durability of the layered structure. If the (003) planeorientation index is shifted to any one of the sides, there is a casewhere characteristics required of a battery cannot be simultaneouslyachieved at a high level so that any one of battery characteristics isunsatisfactory.

As described above, the positive electrode active material includes alithium-manganese-cobalt composite oxide including plate-shapedsecondary particles each obtained by aggregation of a plurality ofplate-shaped primary particles caused by overlapping of plate surfacesof the plate-shaped primary particles, wherein a shape of theplate-shaped primary particles projected in a direction perpendicular tothe plate surfaces thereof is any one of a spherical shape, anelliptical shape, an oval shape, and a planar projected shape of ablock-shaped object, and the secondary particles have an aspect ratio of3 to 20 and a volume-average particle size (Mv) of 4 μm to 20 μm asmeasured by a laser diffraction scattering method.

That is, the positive electrode active material is formed from theabove-described distinctive manganese-cobalt composite hydroxide and alithium compound, and therefore has a structure and characteristicsderived from those of the manganese-cobalt composite hydroxide.Therefore, such a positive electrode active material has an increasedarea of contact with an electrolytic solution and achieves a highfilling density in a positive electrode. As a result, theabove-described positive electrode active material can form a thinelectrode film, which makes it possible to provide a non-aqueouselectrolyte secondary battery that can achieve a high outputcharacteristic, a high battery capacity, and a high electrode density.

(2-2) Process for Producing Positive Electrode Active Material forNon-Aqueous Electrolyte Secondary Batteries

A process for producing the above-described positive electrode activematerial includes at least a mixing step in which the above-describedmanganese-cobalt composite hydroxide and a lithium compound are mixed toform a mixture and a calcining step in which the mixture formed in themixing step is calcined.

The process for producing the positive electrode active material is notparticularly limited as long as the positive electrode active materialcan be produced so that the secondary particles have the above-describedshape, structure and composition. However, the following process ispreferred in that the positive electrode active material can morereliably be produced. Hereinbelow, each of the steps of the process willbe described.

(a) Heat Treatment Step

First, the manganese-cobalt composite hydroxide produced in the abovemanner is heat-treated, if necessary.

The heat treatment step is a step in which the manganese-cobaltcomposite hydroxide is heat-treated by heating at a temperature of 300°C. to 750° C. in an oxidizing atmosphere to remove moisture contained inthe manganese-cobalt composite hydroxide. By performing this heattreatment step, the amount of moisture remaining in the particles untila calcining step can be reduced to a certain level. This makes itpossible to prevent variations in the ratio of the number of metal atomsand the number of lithium atoms in the resulting positive electrodeactive material. Therefore, it is possible to omit this step as long asthe ratio of the number of metal atoms and the number of lithium atomsin the resulting positive electrode active material can accurately becontrolled.

In the heat treatment step, it is only necessary to remove moisture soas not to cause variations in the ratio of the number of metal atoms andthe number of lithium atoms in the resulting positive electrode activematerial, and therefore it is not always necessary to convert all themanganese-cobalt composite hydroxide to a manganese-cobalt compositeoxide. However, in order to further reduce variations in the ratio ofthe number of atoms, all the manganese-cobalt composite hydroxide ispreferably converted to a manganese-cobalt composite oxide at a heatingtemperature of 500° C. or higher.

If the heating temperature is less than 300° C. in the heat treatmentstep, there is a case where excess moisture contained in themanganese-cobalt composite hydroxide cannot sufficiently be removed sothat it is impossible to satisfactorily prevent variations in the ratioof the number of atoms. On the other hand, if the heating temperatureexceeds 750° C., there is a case where it is impossible to obtain amanganese-cobalt composite oxide having a uniform particle size due tosintering of the particles caused by heat treatment. Variations in inthe ratio of the number of atoms can be prevented by previouslydetermining metal components contained in the manganese-cobalt compositehydroxide under heat treatment conditions by analysis to determine theratio between the manganese-cobalt composite hydroxide and a lithiumcompound.

An atmosphere in which the heat treatment is performed is notparticularly limited as long as the atmosphere is one in which reductiondoes not occur, that is, a non-reducing atmosphere. However, the heattreatment is preferably performed in an oxidizing atmosphere, especiallyin an air flow in which the heat treatment can be easily performed.

Further, the time of the heat treatment is not particularly limited, butis preferably at least 1 hour or more, more preferably 5 hours to 15hours because if the time of the heat treatment is less than 1 hour,there is a case where excess moisture cannot be sufficiently removedfrom the manganese-cobalt composite hydroxide.

Further, equipment for use in the heat treatment is not particularlylimited as long as the manganese-cobalt composite hydroxide can beheated in a non-reducing atmosphere, preferably in an air flow, but ispreferably an electric furnace or the like that does not generate gas.

(b) Mixing Step

The mixing step is a step in which the manganese-cobalt compositehydroxide or the manganese-cobalt composite hydroxide that has beensubjected to heat treatment in the heat treatment step (hereinafter,referred to as “heat-treated particles”) and a lithium compound aremixed to obtain a lithium mixture.

Here, the heat-treated particles include not only the manganese-cobaltcomposite hydroxide whose residual moisture has been removed in the heattreatment step but also the manganese-cobalt composite oxide that hasbeen converted from the manganese-cobalt composite hydroxide in the heattreatment step or mixed particles thereof.

The manganese-cobalt composite hydroxide or the heat-treated particlesand a lithium compound are mixed so that the ratio (Li/ME) between thenumber of atoms of metals other than lithium contained in the lithiummixture, that is, the sum of the number of atoms of nickel, manganese,cobalt, and an additive element (ME) and the number of lithium atoms(Li) is 0.95 to 1.60, preferably 1 to 1.60, more preferably 1 to 1.50.More specifically, since the ratio Li/ME does not change before andafter the calcining step, the ratio Li/Me achieved by mixing in themixing step corresponds to the ratio Li/ME of the resulting positiveelectrode active material. Therefore, mixing is performed so that theratio Li/ME of the lithium mixture becomes the same as that of theresulting positive electrode active material.

The lithium compound for use in forming a lithium mixture is notparticularly limited, but is preferably, for example, lithium hydroxide,lithium nitrate, lithium carbonate, or a mixture of two or more of themfor reasons of availability. Particularly, in consideration of easyhandling and quality stability, lithium hydroxide or lithium carbonateis more preferably used.

It is to be noted that the lithium mixture is preferably well mixedbefore calcining. If the lithium mixture is not well mixed, there is apossibility that satisfactory battery characteristics cannot be achieveddue to variations in the ratio Li/ME among the individual particles.

Further, the mixing can be performed using a common mixer such as ashaker mixer, a Loedige mixer, a Julia mixer, or a V blender. The mixingmay be performed to such a degree that the manganese-cobalt compositehydroxide or the heat-treated particles does/do not lose its/their shapeso that the manganese-cobalt composite hydroxide or the heat-treatedparticles and the lithium compound are well mixed.

(c) Calcining Step

The calcining step is a step in which the lithium mixture obtained inthe mixing step is calcined to form a lithium-manganese-cobalt compositeoxide. When the lithium mixture is calcined in the calcining step,lithium contained in the lithium compound is diffused into themanganese-cobalt composite hydroxide or the heat-treated particles sothat lithium-manganese-cobalt composite oxide particles are formed.Further, even when a high concentration layer of cobalt is present inthe manganese-cobalt composite hydroxide, the high concentration layerdisappears due to diffusion so that no structural layered object ispresent.

(Calcining Temperature)

The lithium mixture is calcined at 650° C. to 1000° C., more preferably750° C. to 980° C. If the calcining temperature is less than 650° C.,diffusion of lithium into the manganese-cobalt composite hydroxide orthe heat-treated particles is not satisfactorily performed so thatexcess lithium and the unreacted particles remain or a well-orderedcrystal structure cannot be obtained. Therefore, when the resultingpositive electrode active material is used in a battery, the batterycannot achieve satisfactory battery characteristics. Further, if thecalcining temperature exceeds 1000° C., there is a possibility thatheavy sintering between particles of the lithium-manganese-cobaltcomposite oxide occurs and abnormal grain growth occurs so that theparticles after calcining become coarse and therefore the shape of theabove-described secondary particles cannot be maintained. The thusobtained positive electrode active material cannot produce theabove-described effect obtained by the shape of the secondary particles.

It is to be noted that from the viewpoint of uniformly performing areaction between the manganese-cobalt composite hydroxide or theheat-treated particles and the lithium compound, temperature rise to thecalcining temperature is preferably performed at a temperature rise rateof 3° C./min to 10° C./min. Further, the reaction can be more uniformlyperformed by maintaining the lithium mixture at a temperature close tothe melting point of the lithium compound for about 1 hour to 5 hours.

(Calcining Time)

A holding time at the calcining temperature during calcining ispreferably at least 2 hours, more preferably 4 hours to 24 hours. If theholding time is less than 2 hours, there is a case where thecrystallinity of the resulting lithium-manganese-cobalt composite oxideis unsatisfactory. Although not particularly limited, when the lithiummixture is calcined in a sagger, the atmosphere is preferably cooled to200° C. or less at a temperature decrease rate of 2° C./min to 10°C./min after the holding time to prevent the deterioration of thesagger.

(Temporary Calcining)

Particularly, when lithium hydroxide or lithium carbonate is used as thelithium compound, the lithium mixture is preferably temporarily calcinedbefore calcining by maintaining it at a temperature that is lower thanthe calcining temperature but is 350° C. to 800° C., preferably 450° C.to 780° C. for about 1 hour to 10 hours, preferably 3 hours to 6 hours.That is, the lithium mixture is preferably temporarily calcined at atemperature at which a reaction occurs between lithium oxide or lithiumcarbonate and the manganese-cobalt composite hydroxide or theheat-treated particles. In this case, diffusion of lithium into themanganese-cobalt composite hydroxide or the heat-treated particles issatisfactorily performed by maintaining the lithium mixture at about thereaction temperature of lithium hydroxide or lithium carbonate so that auniform lithium-manganese-cobalt composite oxide is obtained.

(Calcining Atmosphere)

An atmosphere during calcining is an oxidizing atmosphere whose oxygenconcentration is preferably 18 vol % to 100 vol %, and is morepreferably a mixed atmosphere of oxygen and an inert gas. Morespecifically, calcining is preferably performed in the air or an oxygenflow. If the oxygen concentration is less than 18 vol %, there is apossibility that the crystallinity of the resultinglithium-manganese-cobalt composite oxide is not satisfactory.

It is to be noted that a furnace for use in calcining is notparticularly limited as long as the lithium mixture can be heated in theair or an oxygen flow. However, from the viewpoint of uniformlymaintaining the atmosphere in the furnace, the furnace is preferably anelectric furnace that does not generate gas, and may be either abatch-type furnace or a continuous-type furnace.

(Disintegration)

There is a case where aggregation or slight sintering of thelithium-manganese-cobalt composite oxide occurs due to calcining. Inthis case, disintegration may be performed to obtain alithium-manganese-cobalt composite oxide, that is, a positive electrodeactive material.

It is to be noted that disintegration refers to an operation to applymechanical energy to aggregations of secondary particles formed bysintering/necking between the secondary particles during calcining toseparate the secondary particles from each other to loosen theaggregations without substantially breaking the secondary particlesthemselves. This makes it possible to obtain a lithium-manganese-cobaltcomposite oxide having a narrow particle size distribution whilemaintaining the structure of the secondary particles.

As described above, the process for producing the positive electrodeactive material includes a mixing step in which the manganese-cobaltcomposite hydroxide and a lithium compound are mixed to form a lithiummixture and a calcining step in which the lithium mixture is calcined ata temperature of 650° C. to 1000° C. in an oxidizing atmosphere, andtherefore the above-described distinctive positive electrode activematerial can be obtained.

The process for producing the positive electrode active material is easyto perform, achieves high productivity, and is suitable forindustrial-scale production, and therefore its industrial value issignificantly high.

[3] Non-Aqueous Electrolyte Secondary Battery

A non-aqueous electrolyte secondary battery uses a positive electrodeusing the above-described positive electrode active material. First, thestructure of the non-aqueous electrolyte secondary battery will bedescribed.

A non-aqueous electrolyte secondary battery (hereinafter, simplyreferred to as a “secondary battery”) according to an embodiment of thepresent invention has substantially the same structure as a commonnon-aqueous electrolyte secondary battery except that theabove-described positive electrode active material is used as a positiveelectrode material, and therefore will be briefly described.

The secondary battery may have a generally-known structure such as acylindrical type, a rectangular type, a coin type, or a button type. Forexample, when the secondary battery is of a cylindrical type, thesecondary battery includes a case, and a positive electrode, a negativeelectrode, a non-aqueous electrolyte, and a separator housed in thecase. More specifically, the secondary battery is formed in thefollowing manner. A positive electrode and a negative electrode arestacked with a separator being interposed between them to form anelectrode body, the obtained electrode body is impregnated with anon-aqueous electrolyte, a positive electrode current collector of thepositive electrode and a negative electrode current collector of thenegative electrode are connected to a positive electrode terminalconnected to the outside and a negative electrode terminal connected tothe outside, respectively, through a current collector lead or the like,and the case is hermetically sealed.

It goes without saying that the structure of the secondary battery isnot limited to the above example, and the secondary battery may havevarious external shapes such as a cylindrical shape and a laminatedshape.

(Positive Electrode)

The positive electrode is a sheet-shaped member formed by applying apositive electrode mixture paste containing the positive electrodeactive material onto the surface of a current collector formed from, forexample, an aluminum foil and drying the applied positive electrodemixture paste. A product obtained by applying the positive electrodemixture paste onto the surface of a current collector and drying theapplied positive electrode mixture paste is sometimes referred to as an“electrode film”.

It is to be noted that the positive electrode is appropriately processedaccording to a battery in which the positive electrode is to be used.For example, the positive electrode is subjected to, for example, acutting process to cut the positive electrode into an appropriate sizeaccording to a desired battery or a compressing process using a rollpress or the like to increase an electrode density.

The positive electrode mixture paste is prepared by adding a solvent toa positive electrode mixture and kneading them. The positive electrodemixture is prepared by mixing the positive electrode active material ina power form, a conductive material, and a binder.

The conductive material is added to impart appropriately conductivity tothe electrode. The conductive material is not particularly limited, butmay be, for example, graphite (e.g., natural graphite, artificialgraphite, and expanded graphite) or a carbon black-based material suchas acetylene black or Ketjen black.

The binder plays a role in binding the positive electrode activematerial particles together. The binder for use in the positiveelectrode mixture is not particularly limited, but may be, for example,polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE),fluorine-containing rubber, ethylene propylene diene rubber, styrenebutadiene, a cellulose-based resin, or polyacrylic acid.

It is to be noted that the positive electrode mixture may containactivated carbon. By adding activated carbon, it is possible to increasethe electric double layer capacity of the positive electrode.

The solvent is used to dissolve the binder so that the positiveelectrode active material, the conductive material, the activatedcarbon, etc. are dispersed in the binder. The solvent is notparticularly limited, but may be, for example, an organic solvent suchas N-methyl-2-pyrrolidone.

Further, the mixing ratio among the materials contained in the positiveelectrode mixture paste is not particularly limited. For example, whenthe solid content of the positive electrode mixture except for thesolvent is taken as 100 parts by mass, as in the case of the positiveelectrode of a common non-aqueous electrolyte secondary battery, apositive electrode active material content may be 60 parts by mass to 95parts by mass, a conductive material content may be 1 part by mass to 20parts by mass, and a binder content may be 1 part by mass to 20 parts bymass.

(Negative Electrode)

The negative electrode is a sheet-shaped member formed by applying anegative electrode mixture paste onto the surface of a metal foilcurrent collector such as copper or the like and drying the appliednegative electrode mixture paste. This negative electrode is formed insubstantially the same manner as described with reference to thepositive electrode except for the components or composition of thenegative electrode mixture paste and the material of the currentcollector. Therefore, various processes are performed as in the case ofthe positive electrode, if necessary.

The negative electrode mixture paste is a paste obtained by adding anappropriate solvent to a negative electrode mixture prepared by mixing anegative electrode active material and a binder.

Examples of the negative electrode active material to be used include asubstance containing lithium, such as a metal lithium or a lithiumalloy, and an occlusion substance that can occlude and release lithiumions.

The occlusion substance to be used is not particularly limited, andexamples thereof include natural graphite, artificial graphite, acalcined product of an organic compound such as a phenol resin, and apowdery carbonaceous substance such as coke. When such an occlusionsubstance is used as a negative electrode active material, as in thecase of the positive electrode, a fluorine-containing resin such as PVDFcan be used as a binder, and an organic solvent such asN-methyl-2-pyrrolidone can be used as a solvent for dispersing thenegative electrode active material in the binder.

(Separator)

The separator is interposed between the positive electrode and thenegative electrode, and has the function of separating the positiveelectrode and the negative electrode from each other and holding anelectrolyte. As such a separator, for example, a thin film made ofpolyethylene, polypropylene, or the like and having a plurality ofmicropores may be used. It is to be noted that the thin film is notparticularly limited as long as it functions as a separator.

(Non-Aqueous Electrolyte)

The non-aqueous electrolyte is obtained by dissolving a lithium salt asa supporting salt in an organic solvent. Examples of the organic solventinclude: a cyclic carbonate such as ethylene carbonate, propylenecarbonate, butylene carbonate, or trifluoropropylene carbonate; a linearcarbonate such as diethyl carbonate, dimethyl carbonate, ethyl methylcarbonate, or dipropyl carbonate; an ether compound such astetrahydrofuran, 2-methyltetrahydrofuran, or dimethoxyethane; a sulfurcompound such as ethylmethylsulfone or butanesultone; and a phosphoruscompound such as triethyl phosphate or trioctyl phosphate. These organicsolvents may be used singly or in combination of two or more of them.

Examples of the supporting salt to be used include LiPF₆, LiBF₄, LiClO₄,LiAsF₆, LiN(CF₃SO₂)₂, and a composite salt of two or more of them.

It is to be noted that the electrolytic solution may contain a radicalscavenger, a surfactant, a flame retardant, or the like to improvebattery characteristics.

(Battery Characteristics of Non-Aqueous Electrolyte Secondary Battery)

Since the secondary battery having the above-described structure has apositive electrode using the positive electrode active material havingthe above-described distinctive structure and characteristics, the areaof contact between the positive electrode active material and thenon-aqueous electrolyte is increased and the filling density of thepositive electrode active material is increased. Therefore, even whenusing a thin electrode film, the secondary battery can achieve a highoutput characteristic, a high battery capacity, and a high electrodedensity. Therefore, the secondary battery can achieve a high initialdischarge capacity, a low positive electrode resistance, a highcapacity, and a high output while having a thin electrode film. Further,the secondary battery has a high volume energy density. Further, thesecondary battery has higher heat stability and higher safety ascompared to when a conventional lithium-manganese-cobalt-based oxide isused as a positive electrode active material.

(Applications of Secondary Battery)

The secondary battery has excellent battery characteristics, and istherefore suitable as a power source for compact mobile electronicdevices (e.g., laptop personal computers or mobile phone units) that arealways required to have a high capacity.

Further, the secondary battery is suitable also as a motor driving powersource required to produce a high output. In general, a battery becomesdifficult to secure safety as its size increases, which makes itinevitable for the battery to have an expensive protective circuit.However, the secondary battery is excellent in safety, which makes itpossible not only to easily secure safety but also to simplify theexpensive protective circuit to reduce its costs. Further, the secondarybattery can be downsized and can achieve a high output, and is thereforesuitable as a power source for transport machines whose battery-mountingspace is limited.

EXAMPLES

Hereinbelow, the present invention will be more specifically describedwith reference to examples and comparative examples, but is not limitedto these examples. In the examples, evaluations were made in thefollowing manner. It is to be noted that unless other specified, samplesof special grade reagents manufactured by Wako Pure Chemical Industries,Ltd. were used for production of a manganese-cobalt composite hydroxide,a positive electrode active material, and a secondary battery.

(1) Measurement of Volume-Average Particle Size and Particle SizeDistribution

The volume-average particle size and the particle size distribution wereevaluated based on measurement results using a laser diffraction-typeparticle size analyzer (manufactured by NIKKISO CO., LTD. under thetrade name of Microtrac).

(2) External Appearance of Particles

The particles were observed with a scanning electron microscope (SEM)(manufactured by Hitachi High-Technologies Corporation under the tradename of S-4700). The aspect ratio was determined by measuring the aspectratio of each of 20 particles randomly selected by SEM observation andcalculating the average thereof. The average of maximum diameters of theplate-shaped primary particles was determined by measuring the maximumdiameter of each of 50 particles randomly selected by SEM observationand calculating the average thereof

(3) Analysis of Metal Components

A sample was dissolved, and then metal components were determined by ICP(Inductively Coupled Plasma) emission spectrometric analysis.

(4) Production and Evaluation of Battery

(Battery for Evaluation)

A 2032-type coin battery (hereinafter, referred to as a “coin-typebattery 1”) shown in FIG. 4 was used. As shown in FIG. 4, the coin-typebattery 1 includes a case 2 and an electrode 3 housed in the case 2. Thecase 2 includes a positive electrode can 2 a that is hollow and has anopening at its one end and a negative electrode can 2 b placed at theopening of the positive electrode can 2 a. The case 2 is configured sothat a space for housing the electrode 3 is formed between the negativeelectrode can 2 b and the positive electrode can 2 a by placing thenegative electrode can 2 b at the opening of the positive electrode can2 a. The electrode 3 includes a positive electrode 3 a, a separator 3 c,and a negative electrode 3 b stacked on top of another in this order,and is housed in the case 2 so that the positive electrode 3 a is incontact with the inner surface of the positive electrode can 2 a and thenegative electrode 3 b is in contact with the inner surface of thenegative electrode can 2 b. It is to be noted that the case 2 includes agasket 2 c, and this gasket 2 c fixes the positive electrode can 2 a andthe negative electrode can 2 b to prevent relative movement between themto keep the positive electrode can 2 a and the negative electrode can 2b in a non-contact state. Further, the gasket 2 c also has the functionof hermetically sealing a gap between the positive electrode can 2 a andthe negative electrode can 2 b to air- and liquid-tightly cut off theinside of the case 2 from the outside.

(Production of Battery)

First, 52.5 mg of a positive electrode active material, 15 mg ofacetylene black, and 7.5 mg of a polytetrafluoroethylene resin (PTFE)were mixed together, and the resulting mixture was press-molded at apressure of 100 MPa to form a positive electrode 3 a having a diameterof 11 mm and a thickness of 100 μm. The thus formed positive electrode 3a was dried at 120° C. for 12 hours in a vacuum drier. Theabove-described coin-type battery 1 was produced using the thus obtainedpositive electrode 3 a, a negative electrode 3 b, a separator 3 c, andan electrolytic solution in an Ar-filled glove box whose dew point wascontrolled to be −80° C. It is to be noted that the negative electrode 3b used was a negative electrode sheet formed by applying a graphitepowder having an average particle size of about 20 μm and polyvinylidenefluoride onto a copper foil and stamping the copper foil into a diskshape having a diameter of 14 mm. Further, the separator 3 c used was aporous polyethylene film having a thickness of 25 μm. The electrolyticsolution used was a mixture containing ethylene carbonate (EC) anddiethyl carbonate (DEC) in equal proportions and 1 M of LiClO₄ as asupporting electrolyte (manufactured by TOMIYAMA PURE CHEMICALINDUSTRIES, Ltd.).

(Initial Discharge Capacity)

After the completion of production of the coin-type battery 1, thecoin-type battery 1 was allowed to stand for about 24 hours. After theopen circuit voltage (OCV) of the coin-type battery 1 was stabilized,the coin-type battery 1 was charged up to a cut-off voltage of 4.3 V ata current density on the positive electrode 3 a of 0.1 mA/cm², and thenafter a suspension for 1 hour, the coin-type battery 1 was dischargeddown to a cut-off voltage of 3.0 V to determine a capacity, and thecapacity was defined as an initial discharge capacity.

(Cycle Capacity Maintenance Rate)

The coin-type battery 1 was charged up to 4.2 V at a current density onthe positive electrode of 2 mA/cm² and discharged down to 3.0 V, andthis cycle was repeated 200 times to determine a discharge capacityafter repeated charge-discharge cycles. Then, the ratio of the dischargecapacity and the initial discharge capacity was calculated as a capacitymaintenance rate. The charge/discharge capacity of the coin-type battery1 was measured using a multichannel voltage/current generator (R6741Amanufactured by ADVANTEST CORPORATION).

(Rate Characteristic)

The rate characteristic of the coin-type battery 1 was evaluated basedon a discharge capacity maintenance rate at the time when a dischargerate was increased from 0.2 C to 5 C.

Example 1

[Nucleation Step]

In Example 1, cobalt sulfate heptahydrate (Co molarity: 1.38 mol/L) and900 mL of pure water were placed in a crystallization reaction vesselhaving a capacity of 5 L and equipped with 4 baffle plates, and wereheated at 60° C. by a thermostatic tank and a heating jacket while beingstirred at a rotation speed of 1000 rpm with an inclined paddle with 6blades to obtain an aqueous solution before reaction. Nitrogen gas wasallowed to flow into the reaction vessel to produce a nitrogenatmosphere. At this time, the concentration of oxygen in the inner spaceof the reaction vessel was 1.0%. A 6.25 mass % aqueous sodium hydroxidesolution was supplied at 42 mL/min to increase the pH of the aqueoussolution before reaction to 13 on the basis of a liquid temperature of25° C., and then the aqueous solution before reaction was continuouslystirred for 30 minutes to obtain a plate-shaped crystalnuclei-containing slurry.

[Particle Growth Step]

In Example 1, an aqueous solution containing nickel sulfate (Nimolarity: 0.40 mol/L) and cobalt sulfate (Co molarity: 0.20 mol/L), andmanganese sulfate (Mn molarity: 1.40 mol/L) was prepared as a mixedaqueous solution. As in the case of the nucleation step, 25 mass %ammonia water was added to the plate-shaped crystal nuclei-containingslurry in a nitrogen atmosphere so that the concentration of ammonia inthe vessel was 10 g/L, and a 64 mass % aqueous sulfuric acid solutionwas further added to adjust the pH of the slurry to 11.6 on the basis ofa liquid temperature of 25° C. to obtain a slurry for particle growth.The mixed aqueous solution was supplied to the slurry for particlegrowth at 12.9 mL/min while 25 mass % ammonia water as a complexingagent and a 25 mass % aqueous sodium hydroxide solution were suppliedthereto so that the concentration of ammonia was 10 g/L and the pH ofthe slurry was controlled to be kept at 11.6 on the basis of a liquidtemperature of 25° C. to form a composite hydroxide. Then, the compositehydroxide was washed with water, filtered, and dried at 120° C. for 24hours in an air atmosphere. The thus obtained composite hydroxide had acomposition of Ni_(0.20)Co_(0.10)Mn_(0.70)(OH)₂. Further, thevolume-average particle size (Mv) was 10.6 μm and the particle sizevariation index represented by [(D90−D10)/Mv] was 0.65.

The result of SEM observation in Example 1 is shown in FIG. 5. Theaspect ratio measured by SEM observation was 6.3, and the average ofmaximum diameters of the plate-shaped primary particles projected in adirection perpendicular to the plate surfaces thereof (maximum diametersof plate-shaped primary particles projected in a direction perpendicularto the plate surfaces of the secondary particles) was 2.7 μm. Further,the cross-section of the obtained lithium-manganese-cobalt compositehydroxide (secondary particles) was analyzed with an energy dispersiveX-ray analyzer. As a result, it was found that the secondary particleshad a cobalt-containing high concentration layer formed in the center oftheir width direction, and the high concentration layer had an averagethickness of 0.5 μm.

[Production of Positive Electrode Active Material]

In Example 1, the obtained composite hydroxide and lithium hydroxideweighed to achieve a ratio Li/Me of 1.50 were mixed to form a lithiummixture. The mixing was performed using a shaker mixer (TURBULA Type t2Cmanufactured by Willy A. Bachofen (WAB)).

In Example 1, the obtained lithium mixture was calcined at 900° C. for 5hours in the flow of air, cooled, and then disintegrated to obtain apositive electrode active material. The obtained positive electrodeactive material was confirmed to include hexagonal LiMeO₂ and monoclinicLi₂Me′O₃ by analysis with an X-ray diffractometer (X'Pert PROmanufactured by PANalytical). Further, the orientation index of (003)plane determined from an X-ray diffraction waveform was 0.97, and thesite occupancy of metal ions other than lithium in a 3a site determinedby Rietveld analysis was 2.5%. Further, a 2032-type coin battery(coin-type battery 1) was produced to evaluate its initial dischargecapacity, cycle capacity maintenance rate, and rate characteristic.

Example 2

In Example 2, a composite hydroxide was obtained in the same manner asin Example 1 except that a composite solution of nickel sulfate (Nimolarity: 0.8 mol/L), cobalt sulfate (Co molarity: 0.2 mol/L), andmanganese sulfate (Mn molarity: 1.0 mol/L) was used as the mixed aqueoussolution used in the particle growth step in Example 1. The obtainedcomposite hydroxide had a volume-average particle size (Mv) of secondaryparticles of 11.2 μm, the aspect ratio was 5.4, and the average ofmaximum diameters of the plate-shaped primary particles projected in adirection perpendicular to the plate surfaces thereof was 1.5 μm.

Example 3

In Example 3, a composite hydroxide was obtained in the same manner asin Example 1 except that a composite solution of nickel sulfate (Nimolarity: 0.4 mol/L), cobalt sulfate (Co molarity: 0.8 mol/L), andmanganese sulfate (Mn molarity: 0.8 mol/L) was used as the mixed aqueoussolution used in the particle growth step in Example 2. The obtainedcomposite hydroxide had a volume-average particle size (Mv) of secondaryparticles of 10.9 μm, the aspect ratio was 7.0, and the average ofmaximum diameters of the plate-shaped primary particles projected in adirection perpendicular to the plate surfaces thereof was 3.3 μm.

Example 4

In Example 4, a composite hydroxide was obtained in the same manner asin Example 1 except that the pH in the nucleation step in Example 1 waschanged to 13.7 on the basis of a liquid temperature of 25° C. Theobtained composite hydroxide had a volume-average particle size (Mv) ofsecondary particles of 8.7 μm, the aspect ratio was 13.2, and theaverage of maximum diameters of the plate-shaped primary particlesprojected in a direction perpendicular to the plate surfaces thereof was2.4 μm.

Example 5

In Example 5, a composite hydroxide was obtained in the same manner asin Example 1 except that the pH in the particle growth step in Example 1was changed to 10.7 on the basis of a liquid temperature of 25° C. Theobtained composite hydroxide had a volume-average particle size (Mv) ofsecondary particles of 13.7 μm, the aspect ratio was 5.8, and theaverage of maximum diameters of the plate-shaped primary particlesprojected in a direction perpendicular to the plate surfaces thereof was3.8 μm.

Example 6

In Example 6, a composite hydroxide was obtained in the same manner asin Example 1 except that the concentration of oxygen in the inner spaceof the reaction vessel in Example 1 was changed to 3.0%. The obtainedcomposite hydroxide had a volume-average particle size (Mv) of secondaryparticles of 10.3 μm, the aspect ratio was 4.0, and the average ofmaximum diameters of the plate-shaped primary particles projected in adirection perpendicular to the plate surfaces thereof was 2.7 μm.

Example 7

In Example 7, a composite hydroxide was obtained in the same manner asin Example 1 except that the concentration of ammonia in the particlegrowth step was changed to 15 g/L. The obtained composite hydroxide hada volume-average particle size (Mv) of secondary particles of 10.4 μm,the aspect ratio was 5.6, and the average of maximum diameters of theplate-shaped primary particles projected in a direction perpendicular tothe plate surfaces thereof was 2.6 μm.

Comparative Example 1

In Comparative Example 1, a composite hydroxide was obtained in the samemanner as in Example 1 except that the pH in the nucleation step inExample 1 was changed to 12.0 on the basis of a liquid temperature of25° C. The obtained composite hydroxide had a volume-average particlesize (Mv) of secondary particles of 11.3 μm, the aspect ratio was 2.5,and the average of maximum diameters of the plate-shaped primaryparticles projected in a direction perpendicular to the plate surfacesthereof was 1.6 μm.

Comparative Example 2

In Comparative Example 2, a composite hydroxide was obtained in the samemanner as in Example 1 except that the pH in the particle growth step inExample 1 was changed to 12.7 on the basis of a liquid temperature of25° C. The obtained composite hydroxide had a volume-average particlesize (Mv) of secondary particles of 6.7 μm, the aspect ratio was 2.1,and the average of maximum diameters of the plate-shaped primaryparticles projected in a direction perpendicular to the plate surfacesthereof was 0.8 μm.

Comparative Example 3

In Comparative Example 3, a composite hydroxide was obtained in the samemanner as in Example 1 except that the atmosphere in the inner space ofthe reaction vessel in Example 1 was changed to an air atmosphere. Theobtained composite hydroxide had a volume-average particle size (Mv) ofsecondary particles of 3.4 μm, the aspect ratio was 1.3, and the averageof maximum diameters of the plate-shaped primary particles projected ina direction perpendicular to the plate surfaces thereof was 0.4 μm.

Comparative Example 4

In Comparative Example 4, a composite hydroxide was obtained in the samemanner as in Example 1 except that the concentration of ammonia in theparticle growth step was changed to 3 g/L. The obtained compositehydroxide had a volume-average particle size (Mv) of secondary particlesof 7.8 μm, the aspect ratio was 2.5, and the average of maximumdiameters of the plate-shaped primary particles projected in a directionperpendicular to the plate surfaces thereof was 2.0 μm.

[Evaluations]

The volume-average particle size (Mv), the aspect ratio, the average ofmaximum diameters of plate-shaped primary particles projected in adirection perpendicular to the plate surfaces thereof, and thecomposition ratio were evaluated in the same manner as those of thecomposite hydroxide, and the evaluation results thereof are shown inTable 1.

The volume-average particle size (Mv), the aspect ratio, the compositionratio, the non-lithium ion mix ratio in 3a site, and the (003) planeorientation index evaluated in the same manner as those of the compositehydroxide are shown in Table 1, and the evaluation results of each ofthe batteries are shown in Table 2.

In each of Example 1 to Example 7, a lithium-manganese-cobalt compositehydroxide was obtained through a nucleation step in which pH wasadjusted to 12.5 or more on the basis of a liquid temperature of 25° C.to form plate-shaped crystal nuclei and a particle growth step in whicha slurry for particle growth containing the plate-shaped crystal nucleiformed in the nucleation step was adjusted to a pH of 10.5 to 12.5 onthe basis of a liquid temperature of 25° C. but less than the pH in thenucleation step, and a mixed aqueous solution containing at least amanganese-containing metal compound is supplied to the slurry forparticle growth to grow the plate-shaped crystal nuclei. Further, in thenucleation step, nucleation was performed in a non-oxidizing atmospherewhose oxygen concentration was 5 vol % or less, and in the particlegrowth step, the concentration of ammonia in the slurry for particlegrowth was adjusted to 5 g/L to 20 g/L.

The thus obtained lithium-manganese-cobalt composite hydroxide had anaspect ratio of 3 to 20, a volume-average particle size (Mv) as measuredby a laser diffraction scattering method of 4 μm to 20 μm, and aparticle size variation index represented by [(D90−D10)/Mv] of 0.75 orless. A plate-shaped Li-excess lithium composite oxide formed using thecomposite hydroxide as a precursor had the same aspect ratio and thevolume-average particle size as those of the composite hydroxide. Thecoin battery using the Li-excess lithium composite oxide as a positiveelectrode active material was found to be excellent in batterycharacteristics (initial discharge capacity, cycle capacity maintenancerate, and rate characteristic).

In Example 3, the amount of Mn was smaller, and therefore the batterycapacity was lower than those of other examples according to thecomposition, but a high output characteristic and a high cyclecharacteristic were achieved due to the structure of the particles.Further, in Example 4, the pH in the nucleation step was higher and theaspect ratio was higher, and therefore a higher output characteristicwas achieved but the cycle characteristic was slightly lower. On theother hand, in Example 6, the aspect ratio was lower, and therefore theoutput characteristic was higher than those of Comparative Examples butslightly lower than those of other Examples.

On the other hand, in each of Comparative Example 1 to ComparativeExample 4, one of the production conditions of the Li-excess lithiumcomposite oxide was not satisfied, and therefore the obtained Li-excesslithium composite oxide did not satisfy an aspect ratio of 3 to 20and/or a volume-average particle size of 4 μm to 20 μm. Further, each ofthe coin batteries using the obtained Li-excess lithium composite oxideas a positive electrode active material was found to be poor in batterycharacteristics, especially an output characteristic and a cyclecharacteristic.

TABLE 1 Average of maximum Volume-average diameters of Non-lithium ionparticle size primary particles mix ratio in 3a (003) plane (μm) Aspectratio (μm) Composition ratio site (%) orientation index Example 1 10.66.3 2.7 Li_(1.51)Ni_(0.20)Co_(0.10)Mn_(0.70)(OH)₂ 2.5 0.97 Example 211.2 5.4 1.5 Li_(1.51)Ni_(0.40)Co_(0.10)Mn_(0.50)(OH)₂ 2.7 1.01 Example3 10.9 7.0 3.3 Li_(1.51)Ni_(0.20)Co_(0.40)Mn_(0.40)(OH)₂ 2.6 0.96Example 4 8.7 10.2 2.4 Li_(1.51)Ni_(0.20)Co_(0.10)Mn_(0.70)(OH)₂ 2.10.94 Example 5 13.7 5.8 3.8 Li_(1.51)Ni_(0.20)Co_(0.10)Mn_(0.70)(OH)₂3.1 0.99 Example 6 10.3 4.0 2.7Li_(1.51)Ni_(0.20)Co_(0.10)Mn_(0.70)(OH)₂ 2.5 1.01 Example 7 10.4 5.62.6 Li_(1.51)Ni_(0.20)Co_(0.10)Mn_(0.70)(OH)₂ 2.7 1.01 Comparative 11.32.5 1.6 Li_(1.51)Ni_(0.20)Co_(0.10)Mn_(0.70)(OH)₂ 2.9 1.01 Example 1Comparative 6.7 2.1 0.8 Li_(1.51)Ni_(0.20)Co_(0.10)Mn_(0.70)(OH)₂ 2.41.03 Example 2 Comparative 3.4 1.3 0.4Li_(1.51)Ni_(0.20)Co_(0.10)Mn_(0.70)(OH)₂ 3.0 1.15 Example 3 Comparative7.8 2.5 2.0 Li_(1.51)Ni_(0.20)Co_(0.10)Mn_(0.70)(OH)₂ 2.8 1.08 Example 4

TABLE 2 Initial Discharge capacity discharge for 5 C/discharge Capacitymaintenance capacity capacity for 0.2 C rate after 200 cycles (mAh/g)(%) (%) Example 1 261 40.7 64 Example 2 211 39.4 58 Example 3 170 38.866 Example 4 250 43.4 61 Example 5 260 38.5 63 Example 6 260 35.7 62Example 7 258 36.5 64 Comparative 255 29.3 57 Example 1 Comparative 25623.4 58 Example 2 Comparative 254 19.9 51 Example 3 Comparative 257 28.155 Example 4

Glossary of Drawing References

1 . . . coin-type battery, 2 . . . case, 2 a . . . positive electrodecan, 2 b . . . negative electrode can, 2 c . . . gasket, 3 . . .electrode, 3 a . . . positive electrode, 3 b . . . negative electrode, 3c . . . separator

1. A process for producing a manganese-cobalt composite hydroxiderepresented by NixCoyMnzMt(OH)2+A (wherein x satisfies 0≤x≤0.5, ysatisfies 0<y≤0.5, z satisfies 0.35<z<0.8, t satisfies 0≤t≤0.1, Asatisfies 0≤A≤0.5, x, y, z, and t satisfy x+y+z+t=1, and M is at leastone additive element selected from V, Mg, Al, Ti, Mo, Nb, Zr, and W),the process comprising: a nucleation step in which an aqueous solutionfor nucleation that contains a cobalt-containing metal compound so thata content of cobalt to all metal elements contained therein is 90 atom %or more is adjusted to a pH of 12.5 or more on the basis of a liquidtemperature of 25° C. to form plate-shaped crystal nuclei; and aparticle growth step in which a slurry for particle growth containingthe plate-shaped crystal nuclei formed in the nucleation step isadjusted to a pH of 10.5 to 12.5 on the basis of a liquid temperature of25° C. but less than the pH in the nucleation step, and a mixed aqueoussolution containing at least a manganese-containing metal compound issupplied to the slurry for particle growth to grow the plate-shapedcrystal nuclei.
 2. The process for producing a manganese-cobaltcomposite hydroxide according to claim 1, wherein in the nucleationstep, nucleation is performed in a non-oxidizing atmosphere whose oxygenconcentration is 5 vol % or less.
 3. The process for producing amanganese-cobalt composite hydroxide according to claim 1, wherein inthe particle growth step, an ammonia concentration of the slurry forparticle growth is adjusted to 5 g/L to 20 g/L.
 4. The process forproducing a manganese-cobalt composite hydroxide according to claim 1,wherein the slurry for particle growth is one obtained by adjusting a pHof a plate-shaped crystal nuclei-containing slurry containing theplate-shaped crystal nuclei obtained in the nucleation step.
 5. Apositive electrode active material for non-aqueous electrolyte secondarybatteries, comprising a lithium-manganese-cobalt composite oxiderepresented by Li1+uNixCoyMnzMtO2+α (wherein u satisfies −0.05≤u<0.60, xsatisfies 0≤x≤0.5, y satisfies 0<y≤0.5, z satisfies 0.35<z<0.8, tsatisfies 0≤t≤0.1, a satisfies 0≤α<0.6, x, y, z, and t satisfyx+y+z+t=1, and M is at least one additive element selected from V, Mg,Al, Ti, Mo, Nb, Zr, and W) and having a hexagonal layered structure,wherein the lithium-manganese-cobalt composite oxide comprisesplate-shaped secondary particles each obtained by aggregation of aplurality of plate-shaped primary particles caused by overlapping ofplate surfaces of the plate-shaped primary particles, a shape of theplate-shaped primary particles projected in a direction perpendicular tothe plate surfaces thereof is any one of a spherical shape, anelliptical shape, an oval shape, and a planar projected shape of ablock-shaped object, and the secondary particles have an aspect ratio of3 to 20 and a volume-average particle size (Mv) of 4 μm to 20 μm asmeasured by a laser diffraction scattering method.
 6. The positiveelectrode active material for non-aqueous electrolyte secondarybatteries according to claim 5, wherein a particle size variation indexrepresented by [(D90−D10)/Mv] is 0.75 or less, which is calculated fromD90 and D10 of a particle size distribution determined by a laserdiffraction scattering method and the volume-average particle size (Mv).7. The positive electrode active material for non-aqueous electrolytesecondary batteries according to claim 5, wherein thelithium-manganese-cobalt composite oxide is one represented byLi1+uNixCoyMnzMtO2+α (wherein u satisfies 0.40≤u<0.60, satisfies z−x≤uwhen z−x>0.4, and satisfies u≤z when z<0.6, x satisfies 0≤x≤0.5, ysatisfies 0<y≤0.5, z satisfies 0.5≤z<0.8, α satisfies 0.4≤α<0.6, z and xsatisfy z−x<0.6, x, y, z, and t satisfy x+y+z+t=1, and M is at least oneadditive element selected from V, Mg, Al, Ti, Mo, Nb, Zr, and W).
 8. Thepositive electrode active material for non-aqueous electrolyte secondarybatteries according to claim 5, which comprises a hexagonal compoundrepresented by a general formula LiMeO2 and a monoclinic compoundrepresented by a general formula Li2Me′O3, wherein Me and Me′ eachrepresent a metal element other than Li.
 9. The positive electrodeactive material for non-aqueous electrolyte secondary batteriesaccording to claim 5, wherein a site occupancy of metal ions other thanlithium in a 3a site determined by Rietveld analysis of a peakcorresponding to the hexagonal lithium-manganese-cobalt composite oxidein X-ray diffraction is 3% or less.
 10. The positive electrode activematerial for non-aqueous electrolyte secondary batteries according toclaim 5, wherein an orientation index of a (003) plane corresponding tothe hexagonal lithium-manganese-cobalt composite oxide determined byX-ray diffraction analysis is 0.9 to 1.1.
 11. A non-aqueous electrolytesecondary battery comprising: a positive electrode; a negativeelectrode; a non-aqueous electrolyte; and a separator, wherein thepositive electrode is made of the positive electrode active material fornon-aqueous electrolyte secondary batteries according to claim 5.